The game resets or locks up when a particular coil fires, or when the flippers are used.

Reset or lock-up only when one device fires

Cause: A broken or missing 1N4004 diode on that coil (or, as in one Firepower case, on another coil such as a saucer kicker that had also stopped firing).

Checks

• Run the solenoid test and note any coil that does not fire.
• Tug-test the diode on every coil.

Fix

• Fit a new 1N4004 on any doubtful coil, banded end to the lug with the thicker daisy-chained power wire.
• Make sure every coil in the game has a diode.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Game reboots when the flipper buttons are pressed
• Resets from damaged flipper coil diodes
• Testing the diodes on a flipper coil
• Fliptronics flipper coil wired wrongly
• Random resets or lock-ups during play: broken flipper coil diodes
• Game locks up or behaves strangely, worst in multiball
• Testing a coil diode
• Testing the diode on a coil

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)