Sounds like your power supply needs rebuilt. I suggest sending to a pro for diagnosis and fixing.

Power supply = power driver board. Not the transformer, just to be clear here.

Yes. Thank you.

I'm confused here. Is it shutting off and staying off or is it rebooting to the startup screen? You make it sound like it's doing both.

I assume it's randomly rebooting to the startup screen. This is a 5V issue whether it's from a bad connection, failing components or both. With the game on, I'd put a meter on the +5VDC test point and measure what you are getting. Leave the game on and see if the voltage fluctuates after it's been on for a while.

I'd first check the connections on the power driver board at J129 for tarnished pins or connectors. After that I'd test diodes D7 to D10 to see if any are burned or out of spec. Lastly I'd check filter capacitor C9 as part of the +5VDC chain.

If the game is just randomly shutting off (not rebooting) I'd look at connectors in the cabinet, power cord, main fuse or something else also those lines.

Follow the steps at the link below.

https://www.pinwiki.com/wiki/index.php/Williams_WPC#Game_resets

time you send it off for repairs its gonna cost you what a new one would. just buy a new one and be done, its 30 yrs old and power supplies are cheap.