Quoted from Patofnaud:Good place to start...
http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets
+2
This section has been rewritten recently and is very comprehensive. It is excellent. The reason there are so many things to check is because there are so many things that can cause the resets.
This is one of the hardest things to troubleshoot in pinball IMHO. Good luck and don't give up. Check everything.