I have this problem that only appears maybe 1 out of 10 times when I drive my car. When it happens its something like this. I turn the car on the idle is rough the tach was bouncing between 500-1500 rpms almost as if the car is going to stall. If I wait for 30-60 seconds it will sort it self out and then idle smoothly at 1000rpm. So today I didn't wait the 30-60 seconds and popped it into reverse which I noticed to be a bit of a jerk and then I started to reverse without pushing on the gas. However the tach was still bouncing between 500-1500rpm and every time it hit 1500rpm its like the car was pushing on the throttle and making me reverse even faster. SO I hit the brake and every time it hit 1500rpm the only thing keeping the car from accelerating backwards quickly was my foot on the brake. So I pulled out started going forward for about 15 seconds pulled over and put the car in park and the idle was fine at 1000rpm. Not sure where to go from here.

 

What year is it?

 

Its a 99 GLT. And the check engine light hasn't come on.

 

I would try cleaning the throttle body first.