Boy, does this bring back a memory (albeit a hazy one, hope I have all this straight) . . .

Back at my old Indiana arcade, we were shipped a #firepower from another location that did the same thing, and I remember to my surprise happening across a game adjustment it had that controlled background together w/some other function.

Check adjustment function 35. If it's set to 00, 01, 02 or 03, try changing it to 10, 11, 12 or 13 respectively (factory is 12).

(This is on page 4 of the manual; if you don't have one, you can view/download it here: http://www.firepowerpinball.com/2ya/downloads/InstructionBooklet.pdf)

Hope this helps.