2004 BMW 760I Engine Problems

I BOUGHT A 2004 BMW 760I ON 4,23,2013 WITH A 132530 MILES FROM A USED CAR DEALER IN RENO NV , TWO DAYS AFTER BUYING THE CAR I NOTICE WHEN I TURN ON THE CAR IT WOULD IDLE RUFF , THE ENGINE WOULD REV IT SELF AND THE CAR WOULD TURN OFF IT SELF SOMETIMES , WHILE SOME TIME YOU TURN ON THE CAR AND EVERYTHING WORKS FINE THE BIGGEST ISSUES WITH THE CAR IS WHEN YOU PUT YOUR FOOT ON THE GAS YOU NEVER KNOW HOW THE CAR IS GOING TO RESPOND SOME TIME IT MOVES OFF TO FAST SOME TIMES TO SLOW , SOME TIME WHEN YOU STOP AT A STOP SIGN THE CAR TURN IT SELF OFF , THE FORTH DAY AFTER OWNING THE CAR I WAS DRIVING ABOUT 15MPH AND THE CAR TURNED OFF WHILE DRIVING , I TURNED THE CAR BACK ON DRIVE AROUND THE...