Some questions:
1) What work has been done to your boards (if any)?
2) Any aftermarket boards involved?

If there hasn't been any repairs/upgraded boards-in other words everything is all original to the best of your knowledge then there are some things that really need to be addressed. The main things are the 40 pin interconnectors between the MPU and the driver board;if you don't know if they have been recently been serviced REPLACE THE FEMALE CONNECTORS ON THE DRIVER BOARD. It usually not necessary for the male pins on the MPU but it wouldn't hurt to replace them as well. Also on your power supply board replace the capacitors at a minimum- they're most likely WAY past their life expectancy of 10-15 years and probably causing your display issue. Check all of your connectors and repair as needed ; on these older SS pins this is a necessity. For more detailed information about the Williams System 3-7 pinball machines check out Pinsider Vid's guide on them. If you're not comfortable doing your own board work there are folks who can help you with that as well. Good luck with your Firepower - a fun game (and my first pinball as well)!!!