I have a 1999 Chevy 3500 srw with a Vortec 350/410 gears and auto tranny. The truck idles 300 to 400 rpm higher than normal at times. It seems to pulse or rev up on it's own. I had the truck looked at by the dealer 2 times and they can't get the problem to occur while being hooked up to the computer. The throttle body was cleaned and I still have this problem happen several times daily. I think it is something to do with the a/c compressor kicking up the engine idle when the defrost or a/c is used. Does anyone know what could be causing this to happen? Any insight would be greatly appreciated, thanks!