The game locks up at random or will not run.

Random lockups from the 12 V logic pin

Cause: The unregulated 12 V reaches the CPU reset circuit through only one pin each at 1J2 (CPU) and 3J6 (power supply), unlike +5 V and ground with three or more pins.

Checks

• Inspect the 12 V pins at 1J2 and 3J6 for heat damage or poor contact.

Fix

• Replace both headers and their housing pins with Trifurcon crimp pins.

Systems

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

Related tips

• Intermittent lockups with both CPU LEDs on
• Reset or lock-up: which points where
• Usual causes of lock-ups and resets
• CPU never starts: check the reset line at IC1 pin 40
• Game only boots after several power cycles: corrosion under the 6808 CPU socket
• Free play won't take on System 7: do a factory reset first
• Game keeps resetting: replace the 5101 RAM (an NVRAM works)
• Pressing both flippers resets the game: replace big filter cap C15

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