if I try to
accelerate hard the car will go, then it will lose power at about 50 mph no
> matter how much gas I give it.
Check for excessive exhaust backpressure due to internally collapsed or clogged muffler or catalytic converter. Try gently tapping on the exterior with a rubber mallet to see if you hear any rattling inside - not from a loose heat shield but from inside. Then connect a vacuum gauge to the intake manifold, hold the engine speed at a constant around 3,000 rpm, and see if the engine vacuum drops off due to exhaust restriction. I once had a shop volunteer to replace my engine -- a seond opinion revealed an internally collapsed muffler. Different brand of car though, he used a welding torch and somehow from watching the smoke could tell it was blocked. So for the price of a muffler I saved the cost of an engine rebuild.