The mechanics here are confused, and tell me they "think" it's fine. I know it's not.

When it's cold out, and I start the car, the engine revs up for a minute and then drops, suppose this is normal. If I start driving before letting the engine warm up, and say, I stop at a light (xyz for whatever reason) within the first few minutes, and put the car in neutral, the then revs go nuts, up, down, up down, up down, cycles of say 2 seconds. Sometimes, the up down happens even when the car is in gear, i.e driving.

After a few minutes, once the car warms up, this settles down.

Any ideas ?

Many thx.

 

Very common problem, the intake air control valve located at the bottom of the throttle body is dirty. Remove and clean and re-install. That will fix it.

 

Or it's busted & needs replacement.