WHILE IN DRIVE AT IDLE SPEED THE TRUCKS RPM JUMPED TO APPROXIMATELY 4000 RPM INSTANTLY . I HAD TO PUSH MY FOOT BRAKE AS HARD AS I COULD AND PUT SHIFTER IN NEUTRAL THEN IT WAS OK FOR A WHILE THEN THIS HAPPENED AGAIN SEVERAL WEEKS LATER TOOK TO DEALER NO CHECK ENGINE LIGHT COULD NOT FIND A PROBLEM VERY DANGEROUS SITUATION.