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