Quoted from Kazak:it also happens in switch test mode. thanks... i will check diodes
Resets are complicated, I had a similar issue but my problem was the thermistor which is a fairly involved fix.
Diodes are one part of the puzzle but;
Check for any lose connection blots on the boards to make sure they are tight and properly grounded.
Clean and constant inline voltage
Check your 5V is good
This link may help....
http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets