My 1985 Vette has the tuned port injection 350 and when the car isin motion the RPM is around 1500 if you put the car in Neutral....It continues to turn around 1500 rpm until you stop. Once the car sits still for about 2-3 seconds the RPM goes down to 800-900 rpm. Does anybody have an idea what might cause this.

 

Well, on a C5 and C6 models, it would probably be because there is a feature that is programmed into the ECM to keep idle raised a bit, if the car is in motion (even if the gearshift is in neutral), in anticipation that it is about to be shifted into a gear to move again. I'm not certain if there is a feature like that on C4s or not.