mechanics are flabbergasted in 99 chevy venture...need advice

Please HELP!!
This is really long because we have had problems for over a week now and we can't figure out what it is...mechanics are pretty bumfuzzled.
1999 3.4L v-6 4spd automatic

Short version
van started losing power and lights and jerking on interstate. replaced battery and alternator. no light problems now. van jerks really bad, especially going uphill...kinda like not getting gas.
mechanics have checked all good the following- fuel pump, gas, spark plugs, mass air flow sensor. do not think is fuel filter or transmission. no service engine light so no computer codes spit out to help. currently checking resevoir in fuel tank that holds fuel for uphill and downhill.

need help! Does any one have any ideas or advice or help? We are pulling our hair out and this is our only vehicle and we really need it fixed. we live in a rural area and i really don't want to have to tow it again. if you need more info, read long version.

thanks for all help!
Chris

Here is the long version:
On interstate, van jerks slightly, lights dim, and loses power. Goes down to about 30-40 mph and comes back. rides fine for about 4-5 minutres and does it again twice more. we stop at a store to give van a break and figure out what is wrong. Van starts and idles and dies with lights on in parking spot. Started fine again, but died while backing out of space again no lights dimming or going out. we still conclude alternator...no store open at that time, leave overnight in parking lot and come back next day. Afternoon of next day, van would not start at all. Find out changng alternator will involve moving engine, decide to tow to garage. Garage agrees probably alternator. Next day, mechanics got to change alternator, test it first. Alternator tests good. Battery doesn't recharge, mechanics decide it was battery. replace battery with interstate 65 month battery.
Van drives fine up hill and down hill for about 15 miles. Interstate driving again and starts jerking. drove for about 20 miles until jerked so bad could not control. headlights went out. leave parked overnight and take battery out to take home and recharge. We are thinking alternator was bad. Next day put battery back in, use minimum extra/s while driving and take back to garage. Mechanics test alternator and test says good again...still think maybe it tested weird, go ahead and put alternator in.
Van jerks across mountain on way home. Thinking maybe bad gas so run gas to under 1/4 tank add gas treatment and fill with premium. does no good- jerks way home. Although no headlight and dimming problems this time. van stalls going uphill grade over mountain. took 3 tries to get it over mountain and would start after it was turned around and going downhill. It is really bad going uphill.

This is what mechanics have done:
Mechanics have been working for 4 days. They have checked the fuel pressure- good. fuel pump failure not indicated. they are not thinking fuel filter no service engine light and no computer codes to help. the mechanics and us agree probably not transmission by the feeling of it (we've had 2 transmissions go out before). have checked spark plugs and wires, mass air flow sensor, gas checked good, they are currently checking the small reservoir in the gas tank that is supposed to make sure it gets gas uphill and downhill. Don't know if they have checked vacuum lines, fuel pump regulator or fuses, or O2 sensor...not for sure if it would be any of those things.

just for shits & giggles {I} would change the fuel filter//////
it's amazing all the controls go bug-shit when it's plugged
AND NO POWER !!!
fuel pressure test won't tell you //you need to know HOW MUCH
is getting to the engine High pressure with no volume is good
for shit just a suggestion !!!!!

I would consider the possibility of the spark plug wires being worn out. It is a relatively simple task likely to fix this problem. On too many occasions, the basics are overlooked because the vehicle has a computer. NEVER overlook the basics. Ray

Something very similar happened on my 98 venture. No check engine light, just black smoke and no power with lots of jerking. The O2 sensor had gone bad but ws not giving a bad indication. Replaced sensor, and now runs like a champ.

I would be checking the engine ground and body grounds. mainly where the negative battery cable attaches to the block i've seen similar situations where the ground is loose and if the motor move just right(like putting it in reverse) it looses contact and dies but starts fine after a towing. also replace the fuel filter they're cheap and easy to change i recomend changing it every year.
good luck. oh yeah a loose battery ground will kill your alternator too. hope this helps

I have the same problem with my 98 Venture with 85K on it. I have had the O2, Plugs, Plug Wires, Fuel Filter, BG Throttle Body cleaning, Coil Packs. No codes, nothing to indicate a problem.

Not sure what is left to check?

Will the intake manifold gasket cause this problem? EGR, Injectors?

Sounds like more than one person is having this hesitation problem..

Brian

Here is a discussion on subject from elsewhere, hope it helps.

http://www.2carpros.com/makes/pontiac/transpor.htm

The jerking action you are describing would be caused by transmission problems. Problems with such things as the alternator and battery would not cause the vehicle to jerk as you described. Problems related to fuel, spark or timing would not even cause such a problem this would probably be more like a sputtering problem. You have Hydra-Matic 4T65-E 4-speed automatic overdrive transmission. This an electronic transmission the trouble may not lie in the transmission but in the computer, which probably won't give you and diagnostic error codes with a tester because you don't experience the problem while in park. I don't know of any easy way of testing this other than swapping them out and I am not sure which is the cheapest the computer might be so I would start with that. Hope this helps let me know if it does.

I was having some trouble with my venture (sporadic elec. stuff and then it wouldnt start) and had no trouble codes to follow. Luckily I have some connections to a very good mechanic at the Chevy dealer and after spending about 4-5 hours tearing apart the wiring harness in the engine compartment traced it down to a low voltage wite that feeds the computer grounding out. I dont know how he did it because being low voltage all of the fuses were fine. Before I tookit in I checked all of the usual stuff and everything was fine. If I would have taken it somewhere else, it might still be there today!!