Are all the boards in the game original? If so, you're likely due for a bit of upgrades and repairs for the circuit boards. Either way, it's probably a good starting point to post a couple pics of the boards in the backbox. If original boards: do you see any battery corrosion on the mpu board (top-left board in backbox)? It's recommended to replace the big 40-pin connector that links the driver board to the mpu. Does it appear to have been replaced?

Another recommendation I would strongly advise would be to check each fuse in the game. Make sure they're the correct type (slow blow/mdl vs fast blow/agc) and correct amp rating. While you have each fuse out, check them for continuity with your meter to ensure they're good before you put them back in the game.

This is also a good opportunity to add fuses to the 2 bridge rectifiers in the backbox, could prevent a potential fire down the road. Vid1900 has an excellent guide that covers everything I've mentioned and more. If you want to get your game working properly **and safely**, it's well worth the read: https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6

You can also use this tool to help you troubleshoot and repair:

https://pincoder.ca/index.php/2020/07/10/adapter-2020-06-23-0506/

From the sounds of it, the CPU is booting code and the blanking circuit is kicking in causing everything but the GI to turn off. This could be from a faulty components in the blanking circuit, or, more likely, the blanking circuit is doing its job and the CPU is crashing because it cant access code, or that one or more of the RAM/CMOS chips is bad.

The adapter can help you find all the problems much better than the Williams built-in diagnostics.