I PLACED THE VEHICLE IN PARK ON A LEVEL SURFACE, I LEFT THE YUKON RUNNING AND I GOT OUT AND WALKED MY GREAT NIECE TO A BUILDING ABOUT 30 FEET FROM THE VEHICLE.. I THEN TURNED TO SEE THE YUKON BACKING UP ON THIS FLAT SURFACE, I RAN TO TRY AND CATCH IT BUT IT HIT A BUILDING AND THEN CAME FORWARD AT ME. I HAD TO JUMP OUT OF IT'S WAY AND THEN CHASE IT DOWN. WHEN I JUMPED IN AND HIT THE BRAKE I THEN NOTICED IT WAS IN M1. I TOOK IT TO A LOCAL GMC DEALER WHO CHECKED THE TRANSMISSION BUT REPORTED FINDING NOTHING. I AM NOW AFRAID OF THIS CAR. HOW DO YOU KNOW WHEN IT WILL VIBRATE OR JUMP FROM PARK TO REVERSE AGAIN. THIS NEEDS CHECKING INTO. THIS YUKON WENT BOTH BACKWARD & FORWARD. *TR