your game is resetting, it happens in multiball because that's when the most coils are firing to drag the voltage down. it needs to have 5V for the CPU board logic to function but can survive above 4.8 or so i think, anything below that (or whatever the number is) gets detected and it reboots itself so it seems as if the game ended. easiest thing to check is just reseating some connectors. if you can, measure the voltage at the test point on the CPU board, i bet it's only 4.8 now, and that multiball (or sometimes just flipping both flippers together) drags it down another 0.02 or so.
also a mass of other options
http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Game_resets