1995 Buick Riviera 1995 Buick Riviera Loss of power/acceler

I have a 1995 Buick Riviera, 3.8L supercharged, that has suddenly lost power and won't accelerate past about 35 mph. It will only do about 5 mph up hills and frequently stalls. Starts fine though. No engine light has come on, though I did check it and it does work.

I just replaced the fuel filter and the fuel pump. Had the fuel line pressure checked and it is within normal levels. Next, I had the catalytic converter removed from the vehicle, suspecting it was clogged. The car still will not accelerate.

Btw, the following parts were replaced and are new because of problems within the last year, so hopefully it won't be any of them: air temp sensor, coolant temp sensor, alternator, battery, and harmonic balancer.

I'm not a mechanic, but a friend of mine is and is trying to help me, but I think we might have run into a brick wall here.

Anyone have any ideas?