Pretty much has to be environmental, have the power outlet checked. If the B80XX had that issue this forum would be absolutely full of complaints about it and the Xerox support line would be getting absolutely hammered for the last 1.5 years with calls.
So if you have 2 different machines with the same issue that only occurs after a period of time, it pretty much has to be a power fluctuation cooking something in it. Especially since the 2nd replaced the first, which implies it was put in the same spot as the first that had the issue.
We have a B8055 that when making copies off the glass or from the df, they are blank. Prints are good. The solution has been to cycle the power. Once that is done, it works. The dealer has replaced the whole scanner, main board and finally swapped out machine with same model. Well now this loaner one is doing the same thing. Had Canon before with no issues. We have 3 othe Xerox models and the B8055 is the only one doing this. Any ideas?