My '86 305 TPI Z28's idle speed climbs to around 1500 rpm at startup, and stays there for around the first 15 minutes of driving. Idling this high causes the car to really bang into gear when I shift to reverse or drive. However after 15 minutes the idle is back down where it's supposed to be. The engine can be cold or hot - doesn't matter. I've replaced everything I could including the fuel pump. Please help!