I have a 1999 Chevrolet cavalier with a 2.2 engine. It runs great until you get to 40 or 45 mph and then it starts missing and jumping severely but you can accelerate and it smooths out for a few seconds and then starts doing it again. you can drop back down below 45 mph and you couldn't ask for a better running car. PLEASE HELP!
Sounds like an ignition misfire which comes and goes under different loads on the engine. This can often be duplicated by "power braking". Slowly applying the accelerator, while stopped, in gear, wheels chocked, brake pedal firmly applied and no one in front of you.
Once the misfire is duplicated you can short out each cylinder one at a time and determine which cylinder(s) is misfiring.
It can be a plug, a plug wire, or an ignition coil. On this system there are two coils mounted on top of the ignition module, and if one coil goes bad you can lose two cylinders at once (really run bad).
The miss can be coming from other areas, but it's best to determine which cylinder(s) is the problem and follow it from there.