Works from cold, but turned off and back on within a few minutes the LED locks on until the game sits for a while.

MPU locks if switched off and quickly back on

Cause: The reset line is being held up by battery power, usually through corrosion or failed reset resistors such as the two 8.2k parts R1 and R3.

Checks

• Inspect the reset area for corrosion.
• Check R1, R3, R2, R112, R120, R138, R139, R140, R12 and R11.

Fix

• Replace the damaged resistors and clean up the corrosion.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• What each MPU LED flash proves
• Bally/Stern MPU LED self-test: what to expect
• MPU LED locked on with no flicker: likely causes
• Testing the MPU reset circuit at U9 pin 40
• MPU stops before the seventh flash: check the 43 V at R113
• Good MPU stays LED-on with a PC power supply
• MPU boots only when U9 pins 39 and 40 are shorted
• Game reboots during play from slam switch noise

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)