2003 Pontiac Bonneville CYLINDER MISFIRE AND OIL PRESSURE H

Engine Performance problem
2003 Pontiac Bonneville 6 cyl Front Wheel Drive Automatic

MY CAR IS A 03 BONNEVILLE. THE OIL PRESSURE IS HIGH IT SHOWS LOW OIL BUT WHEN I PUT OIL IN IT SHOWS TOO MUCH THEN. MY CYLINDERS 2, 4, AND 6 ARE MISFIRING IN THE ENGINE. I HAVE REPLACED MY SPARK PLUGS, A VALVE, AND THE ECM COMPUTER AND IT IS STILL SHOWING CHECK ENGINE LIGHT ON. AUTO ZONE HOOKED IT UP TO THE MACHINE AND ITS READING CHECK FUEL PRESSURE MAYBE AND OTHER CODES TO DO WITH THE FUEL. I WOULD HATE TO KEEP PUTTING MONEY INTO THE WRONG PARTS. COULD SOMEONE TELL ME IF THEY HAD THESE SIMILAR PROBLEMS AND WHAT THEY DID? IM STILL PAYING ON MY CAR AND CANT GET MY PLATES TILL THIS IS FIXED CAUSE OF EMMISSIONS!