Here you will find the gold standard for tracking such issues down...
https://www.pinwiki.com/wiki/index.php/Williams_WPC#Game_resets
However, I can give you two "quick start" tips.
1. Reseat the following connectors three times each. J101, J115 (both on the PowerDriver board), J210 (on the MPU).
Retest.
2. Jumper across the Thermistor located in the power box, inside the cabinet, right side. The purpose of the thermistor is to limit current to the PowerDriver board upon power up. Resets during the first few minutes are sometimes attributed to this part. This is listed in the link above.
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Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
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http://www.PinWiki.com - The Place to go for Pinball Repair Info