My guess BR2 bridge rectifier.
Easiest fix get http://www.kahr.us/ new reset fix board.

http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets

This is a common incident with this era of machines. The manufacturer installed a "watch dog" chip to protect certain components, so if the voltage dropped below a certain level, the game would shut down. There are many things that could cause this, and as pinballnut3 mentioned, the BR2 bridge rectifier is a common cause. The link pinballnut3 posted above for the small daughter board is a very easy fix that could solve your issue without having to do actual board work repairs. Without getting into all the technical details of how/why it works, it has been a very successful product over the last few months. Buy with confidence!

First guess here as well would be Bridge Rectifier 2.
I have replaced these before and it has solved the problem.

Follow the pinwiki reset guide TheLaw posted before you go replacing things.

+1 better to follow the tried and true process before going straight to replacing board components

The daughter board isn't replacing components, it's cheap, and it just works, so I really don't see any serious downsides. You can always take it off later and try and diagnose, it doesn't permanently alter anything.

Which should start seeing a lot of these with summer here and A/C usage up

Have you tried resetting or cleaning your connections? Sometimes that can improve your connection enough to prevent the voltage drop that's resetting your game.

Hopefully you can find it quick because it can be a bitch.

I am new to fixing pinball machines...recently started helping my grandpa with his Addams Family machine. His machine is matching symptoms above, but it happens whenever he gets a multiball...would this be same process to follow?

does it happen as soon as the multiball starts? Or does it happen during the multiball?

Not right away, and not every time. It just shuts off and reboots randomly sometimes. Most times there's a multiball in the last two days though, it has shut off. It didn't happen before I changed the batteries for the first time in 6 years. He wasn't aware you needed to do that.

I had this problem for a while. It wasn't the multiball itself causing it, it was the players' natural increase in activating all flippers at once that was causing my resets. I reseated the connectors on the power driver board, which stopped the resets. However, this is only a temporary fix, as it probably indicates an issue with the 5v supply.

The daughterboard mentioned above is a quick fix, but if you have soldering experience, replacing the bridge rectifier is probably the fix. I haven't done it myself, even though I've purchased the parts, because when I removed my board, I found that a previous owner had 'tried' to perform this repair and lifted several traces. I wasn't in the mood to attempt a repair only to have the mostly lifted trace become a completely lifted trace.

I just solved this behavior on mine about 3-4 weeks ago, took me a full year to trace the problem in a systematic fashion (work, being PCS'd, 14 hour days, etc. my time was limited).

The game would reset at totally and completely random times: multiball, start of MB, start up of the game itself, skill shot, Thing making his shot, ball going down the ramps, pop bumper activity, slingshot activity....it drove me nuts.

I read the Pinwiki, hit everything in order up to board components. I saw it this way, new components, a proper desoldering gun to keep the traces safe, shipping, etc. was about $20 shy of a new board, and I could sell my old board to someone trusted in the community to fix it and keep it in our community, so I bought a new RD power driver board and nuked all that shizznazz in one shot (and ended up getting $150 for my new board, so I was only out $150, and a new desoldering gun would be $200+ at a minimum).

Plus, on top of that, I didn't have to worry about swapping out ALL those male header pins on the board for the 5V connectors I am about to mention...

Game was much better, but it still happened here and there. Next thing to hit was any IDC connector that was responsible for carrying 5V supply. Fellow pinsider, Schudel5, nailed it here:

https://pinside.com/pinball/forum/topic/reset-problem-again#post-3638849

Now, like I said, the board has been swapped, so you know all your male pins are good, so now it's time to swap out ALL the female connectors: J101, J102, J114 (on PDB) and J210 (on CPU).

That nearly finished it off, but I still had a reset issue here and there. Went back to the guide and read up on the thermistor in the power box. Swapped it out with a new one, and so far......so far.....the game has been a tank.