6400 CQ & scanning issues

This thing has intermittent black copies, from anywhere: DH, glass, or print. Black drum cartridge replaced - no luck. My guess is the drum is loosing charge, but how?
The second problem is that the DH scans (to file) sometimes have a yellow background that shows up on their computer. I have not been able to determine if scans from the glass exhibit the same issue of yellow background. Trying to link the two problems of black copies and yellow background on scans. Any ideas? LVPS, HVPS, etc? Hopefully some of you smart people have seen this problem before and can offer a tad of advice.

Thankx in advance, Glenn