Then have someone look at it physically. Sounds like you need to find where the toner is spilled/leaking internally.
I'd try having someone diagnose it on site, don't want to keep just throwing money at the problem.
Next thing on the money path is the Transfer Roller
Open a blank, one page document.Send 10 copies of the blank document to the printer.
If the output (print) of the blank document displays background and the background does not fade or get considerably lighter after the 10 copies are made, then the Imaging Unit should be replaced.
If the background does fade or lighten, or is not displayed at all, continue with normal printing.
Almost every time I print a documenti, it covers the background with a gray pattern. Sometimes it also do it in other colors.
What could it be ?