2005 CADILLAC XLR Steering Problems

WHILE DRIVING MY 2005 CADILLAC XLR ON THE INTERSTATE HIGHWAY AT APPROXIMATELY 70 MPH, DRY HIGHWAY MY CAR DIPPED TO THE LEFT WITH INSTANTANEOUS STABILITRACK ACTIVE FLASH. THEN A FEW SECONDS LATER DIPPED LEFT AGAIN AND PULLED SEVERALLY TO THE LEFT. THE CAR WAS OUT OF CONTROL. I COULD FEEL THE ABS PULSATING IN THE BRAKE PEDAL. FORTUNATELY NOTHING WAS IN THE LANE NEXT TO ME OR THERE WOULD HAVE SURELY BEEN AN ACCIDENT. I LUCKILY WAS ABLE TO GET THE CAR BACK INTO MY LANE BY JUST BEING FULLY AWARE OF MY SURROUNDINGS AND HAVING BOTH HANDS ON THE STEERING WHEEL AT THE TIME. A FEW SECONDS LATER ANOTHER QUICK DIP AND THE SERVICE STABILITRACK CAME ON. NO FURTHER EVENTS. I CALLED ONSTAR FOR DIAGNOSTICS AND NO TROUBLE CODES WERE FOUND ALTHOUGH THE SERVICE STABILITRACK WAS CLEARLY ILLUMINATED. AFTER A 5 MINUTE STOP (ENGINE OFF/RESTART) THE SERVICE STABILITRACK DID...