Terradyne Kappa

99 silverado 5.3L, 140k miles; problem just started. After the engine gets warm RPM's run high when idle. At stop light I had to force it to stay still. I checked the throttle cable and it is fine. It comes back to normal position. Here is the trick, when I shut it off and start it up again the RPM's are fine until I touch the gas pedal. It's anoying. Anyone with same problem ?