THE TRUCK WHEELS LOCKED UP AND ENGINE SHUT OFF WHILE TRAVELING AT APPROXIMATELY 40 MPH, COMING TO A VERY SUDDEN STOP. TRUCK WOULD NOT RESTART FOR ABOUT 5 MINUTES. DURING THIS MALFUNCTION THE TRUCK IS COMPLETELY UNCONTROLLABLE. THIS HAPPENED TWO MORE TIMES AND WAS AT A DEALER FOR DIAGNOSIS FOR SEVERAL WEEKS. UNABLE TO FIND ANYTHING, IT WAS RETURNED TO US. THE SAME THING HAPPENED AGAIN, THIS TIME CAUSING AN ACCIDENT BECAUSE THE TRUCK HIT A GUARD RAIL.