IDLE AIR CONTROL (IAC) VALVE STUCK, CAUSING SUDDEN ACCELERATION AND CRASH INTO A BUSINESS SIGN. ASSUMED DRIVER ERROR SINCE NO OTHER APPARENT CAUSE. AFTER REPAIRS, ON JANUARY 24, 2000, THROTTLE OPENED SUDDENLY AGAIN CAUSING CAR TO ACCELERATE IN REVERSE WHILE BACKING FROM RESIDENCE GARAGE. NO DAMAGE THIS TIME. LATER SAME DAY AT HIGHWAY SPEED, THROTTLE AGAIN OPENED BUT WAS BROUGHT UNDER CONTROL. *AK