I have a restored Dr. Who pinball machine. I have had some recent repairs done and now, the unit is restarting. I had thought that it was random but, I have found that if I hit the flipper switches simultaneously the system restarts pretty predictably. At first I thought the power reset was related to the mini-playing field but, I think you are just more apt to use both flippers at the same time when the mini-player field releases both balls.
While I wait to hear from the forum, I also plan to buy a UPS just in case it is a power quality problem.