I would use the pinwiki approach for typical reset issues. However, since it left working I would start with your wall voltage.

Could be any number of things... here is a good read:

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

Burnt connectors have always caused this problem for me. I would check at j101 and 102 connectors.. If they are brown that means they need to be replace both board side and off board. Also j114..

1.) Check your wall voltage. 120VAC or close?
2.) Does the problem occur when you hit JUST the left flipper? I ask this because the error you are seeing is significant. Go into the switch test mode, hit the right flipper and you should see a switch toggle for both the lower right EOS switch and the upper right EOS. The flipper might appear to be functioning properly, but if the EOS switch is broken or not making contact, then you are drawing an awful lot of extra current/voltage through the flippers.
3.) Check the main boards for burnt connectors. J101, 102...and I think 114 or 115 are common culprits.

Recently had reset issues on my TAF. Booted fine, but would reset when both flippers were activated at the same time.

The main filter caps were bad, well 3 of the 5. The worst one measured at 10,800uf .

tighten the grounding / mounting screws that hold the driver board in place . . .

Check connector J806 on the Fliptronics I board in the upper left hand corner of the backbox.

RM

WPC_Fliptronics_I_Board.JPG

I would also reset all your ribbon cables . Generally the ones that go from the CPU board.

Don't know about reset, but need to check wires at all flippers for eos error.

reseat all connectors on the power board...I cant look right now but j114 0r j109 0r j102 0r j110 ring a bell but reseating all of them should solve it. I had this happen to my ij and tz at different times but reseating two molexes solves it all the time for me. I just cant remember which two molexes they are....I'll see if I can find the paper I wrote it down on.