I replaced the voltage regulator chips on the power board and now it provides the ideal output. But the game still claims that a RAM chip is bad (1-3-1 error on the ROM board). That could be because of a bad power supply, but I'm pretty sure the power is good. And I know the RAM is good since I just replaced every chip and checked the voltage on each one.
Maybe there is a logic error on the motherboard. I don't have the tools to test each chip on the motherboard, so I am stuck. If only I had a known working motherboard...
EDIT:
I found someone sort-of-local that knows how to repair Williams games. He knew how to find the bad chip on the motherboard. After that, the game worked fine.
