Yep. The 7900 is total not-good go printer. There is a light small sensor about 5 inches from the right side of the printer right as the paper goes into the printer. It has to see white paper.
Putting a printed page in there and then gently holding a white sheet over it (but not resting on the rollers), then hitting print, will possibly fix this issue.
Dana and I are working on a hack. It's been awhile with this printer . . . .