1999 BMW 740 acceleration problem

HI, MY 1999 740IL HAS A PROBLEM WITH IT'S ACCELERATION. WHEN GAS PEDAL IS PUSHED 10% DRIVES OK, 50% SHE STOPS ACCELERATING, CHOKES, AND BACKFIRES BUT WHEN PEDAL IS PUSHED 100% IT COMES BACK TO LIFE AND ACCELERATES GOOD WHILE BEING PUSHED 100% DOWN THE WHOLE TIME. (WORK DONE) REPLACED SPARK PLUGS WITH NEW BOSCH PLATINUM+2'S, AND ADDED MULTIPLE FUEL CLEANERS AND PROBLEM STILL EXIST'S. ALSO WHEN CHANGING PLUGS THEIR WAS SOME OIL IN PLUG AREA, GUESSING LEAKING VALVE COVER GASKET, BUT DON'T THINK THAT IS THE WHOLE CAUSE OF PROBLEM JUST PART OF IT. WHAT DO YOU THINK??