You could have a bad transistor on the flip board, or a shorted diode on your coil, or, as happened to me, a screw that was too long that is shorting out an un-wrapped coil, or a shorted coil itself.
I'd start by checking the coil and diodes first then move upstream from there to the flip board.
If you are new to this go to the pinwiki resets it is super helpful.
http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets