Hi, this is for my 98 5.7 V8 z28 with 137000 Km. Been noticing a recurring problem - rise in the idle speed of 13-14 on the rpm meter, often when I stop momentarily before the light turns green, this leads to a hard jerk forward as I put it on drive. Idle speed returns to normal once engine is restarted. Can some one tell me what it can be & how to diagnose it? Thanks.

 

It could be you IAC (idle air control valve). I would take the throttle body off and clean the IAC out or just put a new one on.