If you can get it in the "failed" state, squeeze the EOS switch contacts closed tightly while holding the flipper button in. If that makes it fire, the EOS switch is dirty or bad.
Another thing I've encountered is when the gold plating on the flipper button switch gets worn away, the switch becomes intermittent. New switch at that point, but make sure you use a good plated switch; the old style (tungsten?) Switches will seem to work but become intermittent soon.