Driveability Problem 98 Merc GM

I have a problem with my GM-same as Crown Vic. The car starts and runs great at idle. It also runs great until you reach 30 to 40 MPH. It bogs down at this speed range and will not go any faster. Acts almost like it is not getting enough fuel. New fuel filter installed. Fuel pressure at idle is 30 PSI. If you rev engine it increases to 34 -35 PSI. Engine will rev to red line in neutral and runs smooth. No check engine light or codes in PCM. Exhaust is not plugged-Cats are good. Dropped exhaust system at manifolds to check-did not make any difference in how car ran. Engine bog down appears to be speed sensitive-not RPM sensitive. When i put tranny in 1st gear the engine bogs down at 30-40 MPH. When I put tranny in 2nd gear bogs down at 30-40 MPH. In drive it bogs down at same 30-40MPH. When it bogs down the car will not accelerate even if you slow down to 15-20 MPH. If I drive between 20-25 MPH car runs great and will accelerate if you slow down. It will run great for long periods of time with no problems if I do not go over about 25 MPH. When engine bogs at 30-40 MPH it has a slight miss like it is not running on all 8 cylinders. I have been fighting this for about 3 weeks. 1 or 2 times when I took it for a test drive it has had a small backfire when it bogs down. Very rare to backfire considering number of times I have taken for test drive after checking various items. Cleaned MAF sensor and also test drove with it unplugged. Did not help or change 30-40 MPH bog down problem. Thanks in advance for any help with my problem. Dan Email address- corv1947@aol.com.

Are all spark plugs firing? In the past, I had a Coil-On-Plug fail on one of my cylinders which caused the engine to surge. Although mine ran rough at idle, too.

When you are at the 35-40MPH limit, is your engine near max RPM? If that's the case it sounds like it definitely has to do with the transmission not shifting into higher gears. 2nd gear would max out around 35-40MPH. The 4R70W transmission you have may have a bad solenoid or blocked ATF passage way preventing proper shifting. Also since the transmission is computer controlled instead of mechanically, there may be a problem with the computer shifting the transmission due to a bad speed sensor.