Well that might be your problem right there, a plastic head that old with that much use on it is bound to have a lot of wear and tear, and develop slop, that can easily contribute to blade tracking issues. Just a thought, maybe i am wrong altogether, but check all your ball links and be sure they are snug and not sloppy, as well as checking to see if the washout and swash bushings and bearings are all still slop free.