vibrations over 65 mph or downhill

Ihave a 98 pontiac bonneville. I am at my wits end. My husband took it today to the 7th person to look at. I am desperate. I am so sick of getting charged and getting nothing accomplished. My car started vibrating when going over 65 mph. I just assumed because we live on a rough road that it was the tires out of balance. This is exactly what it feels like. Got them balanced 3 different times to no avail. I bought new tires. Was told they were slightly out of round. Bought new tires again. Did no good. I was told maybe it was a bent wheel. I got new wheels. One mechancic that said the wheel was bent charged me 50.00 to put the front wheel in the back and say it was the wheel. I was told it could be the Torque converter. I paid to flush out the transmisson. Still no help. I have been told it was - or could be- harmonic balancer, cv axle, wheel bearing, struts, wheel, etc. I have also been told it was not these things . Each mechanic says something new. I cannot keep putting money in this car and not getting anywhere. I now have in it almost what I paid for it. Mechanically,its been a great car. It has 90000 miles on it. It does not vibrate noticeably until over 65mph or when going downhill. It does this whether you break or not. Breaking does make it worse, though. Took it today to the pontiac dealership (not the place I bought it from) They said it was the transmision needed rebuilt and said trans, and motor mount is broken. What I cant figure out is, it does this, even if you drop it in neutral. It does not matter if you accelerate or not, or even if its in gear. I already had a trans man tell me the trans was fine. What should I do? I am thinking about just having them do the mounts first, and then the trans if that doesnt work. It does not make sense to me that its the trans .if it does this in neutral. I have been screwed so much at this point I dont know what to do. They are a reputable dealership, serv. dept, but are they just trying to make more money off me.? What could be causing my shaking under these circumstance? Any help will be appreciated. I am at my wits end and I feel like it would be a mistake to keep throwing money in it. However, I could not and would not sell it like this until it was fixed anyway. They car is in exc shape otherwise, but it is a 98 model with over 90000 miles. I have been to so many mech. trying to screw me and noone really seems to know. Is it possible that it is the transm. and motor MOUNTS ? Should I see if that fixes it before I pay for the trans work? Would this make a car shake over 65mph? I no longer know a mechanic I trust. I know there are some good ones out there, but I cant find them. Even the best will gurantee the work they do, but wont guarantee that the thing they fix will be the problem. They all contradict each other. I cant keep putting every penny I have and not even fix the problem .Please help. I having serious stress over this.

not having seen the car, I would hesitate to diagnose this problem, but from what you say, i would say teh transmission output shaft bearing has failed and is allowing the cv axle to oscillate. Since the cv axle is directly connected to the wheel, it would vibrate whether the trans is in gear or not. Since the trans mounts are bad, I would say that they went bad as a result of this oscillation, and are not the cause of it. Have you noticed any trans fluid leakage, even a very small amount around where the cv axle enters the trans? If so, then that would be my guess. If there is no leakage around the trans, then it's very possible you have a brake rotor that's out of balance, not necessary warped, but not properly balanced when produced causing a high-speed up and down vibration similar to an out of balance wheel.

Vibrations are not that difficult to diagnose, it just takes some math and the right tool.
Personally, I like doing vibration diagnosis, not many techs do or are proficient at it so I have made it my nitch around the shop.
The problem is they are not trying to screw you; they just don’t have the tools or experience to properly diagnose a vibration concern.
Every GM dealer was sent an EVA (electronic vibration analyzer). This is a tool that will display the order of vibrations (order is the # of shakes per revolution of a component or component group).
Also in GM Service Information there is a vibration diagnosis table and worksheet that will assist in making the determination on what order and component group are causing the vibration.
There are some particular road tests that need to be performed to determine if your vibration is related to 1 of 2 scenarios (Vehicle speed related vibration or an Engine speed related vibration)
Certain conditions cause certain order vibrations that can be directly linked to a component group (operational characteristic of a component will cause a certain order vibration)
One thing I would definitely look at is the mounts. If they are damaged, that could be the cause of your vibration. Something could have caused them to fail or they have just disintegrated over time.
But a proper roadtest and inspection will uncover the source of your vibration.

Thanks for your reply. I will ask the Pontiac Service department if they used the EVA. It is so frustrating because I know nothing about cars and I feel like with so many mechanics , you have to already have an idea of what it could be, or know what questions to ask . I know a lot of people that work on cars that can do most basic things, its just so few of them seem to be able to diagnose the problem.(kind of like a doctor) I dont go much , but when I do, I'd hate to think I had to go in and tell the doc that I think I need my gallbladder out. I kinda hope he's the one that figures it out. As far as trying to screw me, I dont think at all think that everyone is. I just think many are and if they dont know , they should just say so. As my earlier post said, one guy put my front wheel on the back and told me it was a bent rim. He then proceeded to charge me $50.00 for something he did in 10 minutes and he was wrong.(lawyers dont make $50.00 every 10 minutes.) I seem to be getting closer to finding out by searching and posting in these forums. I put the car in neutral the yesterday and held the break. When I rev up the engine hard I notice it shakes also. (but only when i really hit the gas hard. )But I was not sure if any car wouldnt do this under the same circumstance. I am just really worried because I would hate to spend $1400.00 on the trans. and leave that parking lot with it still just doing the same thing. Maybe I will just have them fix the mounts first ($250.00) . The other poster mentioned that it could be the transmission bearing. I wish I could communicate as well with the shop mechanics as I can with people on the forum. I just cant seem to get a whole lot of feedback with some of them. Thanks everyone for the replies. Ill feel better if I can at least eliminate what it isnt. thanks

If you shut the engine off and coast down the hill and it vibrates I would look at inner cv joints or possibly the tires need a road force variation balance done, but that would be 2 sets of tires doing it, unusual. GMmerlin's advice is on target, but most dealerships don't want to spend the time/money to perform these tests. Careful if you try what I suggested. No brakes and steering shutting the engine off!!

I have a 94 Bonn and had a lot of problems with
vibrations to, it's going into the shop for new inner
tie rod ends driver side.