Lift the glass off, go into multiball mode by finger-triggering the appropriate switch/target to go into multiball mode, then finger each of the targets & switches until you can re-create the fault of shutting down, you MIGHT also have to press multiple targets at once to simulate multi-balls hitting them to try and find what triggers the shutdown.

Besides the above, just run anormal game, and finger every target/switch to see if 1 switch alone is causing the short/shutdown......it may (or may not) be unique to multiball mode...

MM.