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

Since this is probably the easiest, measure the coil resistance first and see what you get.

Next, lift up the playfield and look at what happens when the flipper moves, and like buffaloatx said, something may be touching something else when it's not supposed do.

Next, check the transistor with a multimeter. Also check the connector to make sure there aren't any wires popping out or any stray copper hairs from the end of a wire touching neighboring wires. Also check the ribbon cable to make sure the insulation hasn't worn through near the ribbon cable connectors.

If all that seems to check out, then check and/or replace the diodes.

I usually try to troubleshoot according to a combination of what is easiest to check and rule out, and what is most likely to cause a problem.