We have 2 printers, so we took the entire belt assembly and belt out of one and put it the other. We also replaced all the drums, the developers and powder.
We think it maybe something organic like in the laser assembly or the ROS. We haven't cleaned the mirrors yet, that is our next step.
The printer is almost brand new, but it sat in our garge with no heat or air for 5 years.
Sounds like you have tried everything already, so it would be a hardware escalation: 1-800-821-2797.
I have a Xerox 7775 and it's printing these strange gray bullseye (circle) spots all over.
I changed out:
All the Drums
All the Developers
The first 20-30 sheets are fine, then as the printer heats up, the spots start showing up, getting progressively more pronounced as more pages print. The spots are only black.
After changing everything, the spots are much, much lighter with fewer showing up. The spots that show after the change of parts are in the same locations as before changing the parts.