'99 Silverado idle & AC problems

I have a '99 2500 5.3. After the engine is warm and I come to a stop, the idle revs from 700 to 1200 rpm and knock - after about 5 secondsit will stop revving until the next stop light. It will also stop revving if put in neutral. I've had it to a good mechanic and he believes in the intake manifold gasket - but the engine doesn't run hot . . . ? Iv'e also been told to check the vacuum line leading to the fuel pressure regulator. Any advice or confirmation of the above diagnostics?

I'm not sure if this is related or not to the above . . . sometimes, especially when towing my boat, my AC will make a buzzing sound like an electric razor. Turn off the AC and it will go away. I've been told the compressor is fine. Any thoughts.

- Former happy Chev guy

Replace your spark plugs wires ..i had the same problem with my truck i replaced them twice due to hot weather and heavy use ... and gane away try it before you do anythings eles :screwy: