I had the same thing happen to my Williams Getaway HP2, mine would re set if the balls where in the supercharger and both flippers energized.
Wiggled the J10 connector on the driver board (top right) and that fixed it for a time but it came back.
I had the pins replace at J10 and one of the big black caps replaced and the problem went away.
This...the machine is drawing too much power during the multiball mode dropping it below the minimum for the power driver board to operate or the current going in must be only just above the minimum.
In addition to the above, plug the machine directly into the wall socket, without using an extension cord and don't have any other appliance sharing the same line.
