HP Laserjet 8000DN

I keep getting either a 50.1, 50.3 error, or every once in a great while I’ll get a couple of pages that don’t fuse at all, but continue printing after those pages with no problems. I have replaced the fuser three times, the DC controller, the low voltage power supply, all to no avail. PLEASE don’t sent me any pat answers, as I guarantee they will be turned down out of hand. I have used the service manual and what little HP’s website has to say about troubleshooting this problem. Thanks for your help.