I am having the same problem. It seems to be the throttle body switch. I know that my switch is not making good contact when the throttle is in the idle position. When the contact opens, the idle controller stops regulating the idle speed and it creeps up. Then due to vibration or some other phenonemon, the switch makes contact and the idle controller regulates the idle properly. If I quickly press the gas pedal and quickly release the idle returns to normal, sometimes, or not.
You can check by using an ohm meter to measure the resistance of the contacts. The plug for this switch is located just under a PCV hose very close to the cold start injector valve. Move the hose and unplug the connector. There are three wires. Center is common and the one contact is made then the throttle is fully closed, and the other when the throttle is fully open. Make sure that you get a good steady low resistance reading at each throttle position.
If thats not it you might have problems with the idle control valve. Its that cylinder shaped device just to the left of center on the top of the engine. It vibrates when the engine runs. They get gunked up with goo and need a cleaning from time to time. I would put money on the switch if I were a betting man.
Good luck. Let me know what you find.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k