Hi, I have a 2001 Cavalier VLX four-door five-speed with about 65,000 km on it. It has the original spark plugs, fuel filter, plug wires, etc. I've been keeping up with the oil changes regularly. The car has the 2200 SFI (LN2).

When I am driving the car normally - and this has always been the case since I've owned it - there is a lot of vibration in the gas pedal, almost like the car is compression braking. I'm in the appropriate gear for the speed and if I let go of the throttle, the car significantly compression brakes. The throttle response is quite laggy, and it feels like there's a lot of "throw" in the gas pedal - I have to push the pedal down quite a bit to make it accelerate.

However, as my stick driving has improved, I've noticed that if I get a perfect launch and continue upshifting smoothly, the car's throttle response is crisp and the vibration in the gas pedal is gone. If I let off the gas and let the car drop to or below 2000 rpm, the car goes back to the "laggy" feel and it is difficult to accelerate from the middle of the rev range. If I stop the car and wait for a while at idle - eventually the tachometer 'jiggles' slightly after waiting a few seconds in neutral - and then launch again, the crisp throttle returns.

Obviously at some point I have to drive below 2000 rpm - I am wondering what the source of this problem is. The slow launches are frustrating and downshifting on the expressway when someone gently brakes in front of me is frightening.

Originally I thought it might have been clutch/flywheel related but that part of the car seems to be fine. The only other thing I can think of is fuel delivery or spark - somehow the computer seems to be changing a setting (perhaps retarding spark? I'm not up on my engine theory) when it drops below 2k.

Also, I should point out by "perfect launch" that I just mean that I start the car at 2500 rpm and apply throttle/clutch properly to maintain the rpm instead of letting it drop and 'lug' the engine. I am not doing high RPM launches and never have.