VEHICLE UNEXPECTEDLY GAINED SPEED WHILE BACKING OUT OF DRIVEWAY CAUSING COLLISION WITH MAILBOX BEFORE CAR COULD BE BRAKE TO STOP. PRIOR TO THIS THE VEHICLE SPEED WOULD REMAIN CONSTANT WHEN FOOT WAS REMOVED FROM ACCELERATOR PEDAL WHILE DRIVING AT APPROXIMATELY 35MPH THEN THE CAR GAINED SPEED ON ITS OWN. VEHICLE WAS ABLE TO BE BROUGHT UNDER CONTROL BY BRAKING EVEN AS THE IDLE SURGED. VEHICLE WAS TAKEN TO AUTONATION FORD OF KATY WHERE THEY DIAGNOSED "ROLLING IDLE FROM PROPORTIONING VALVE LEAKING AT BACK OF INTAKE" AS THE CAUSE. WORK WAS NOT PERFORMED BECAUSE OUR SON WHO HAPPENED TO BE IN TOWN SAW THAT THE CRUISE CONTROL CABLE HAD BECOME DETACHED FROM THE THROTTLE. MY SON REATTACHED THE CABLE AND THERE HAVE NOT BEEN ANY FURTHER ISSUES WITH A STUCK IDLE OR UNATTENDED ACCELERATION. *TR