Make sure there are no other magnetic fields near your monitor like subwoofers, television/stereo speakers, etc. Heck, I've even seen instances where a metal game control panel got magnetized on an adjacent game which affected the monitor (rare, but it does happen). If you still can't get your picture perfect, rotate the cabinet 90 degrees and degauss it again. Sometimes you have to move the game to another place in the room to correct it.