take off your air filter and confirm that the carb is actually closed. Sometimes the throttle linkage in the carb will need to be repaired: it will seem fully closed according to your servo but it actually isnt moving the inside of the carb itself.

Only other thing it could be is for some reason you might be running insanely lean.. like when your truck is ideling and is starting to run out of gas and revs up through the roof before it dies.