Credits appear on their own every few minutes.

Game adds credits by itself

Cause: A lamp socket shorted by conductive flux residue upsets the switch matrix through the shared strobe lines.

Checks

• Run the switch test and see which switch closes by itself.
• Look along that strobe's lamp sockets for heavy flux.

Fix

• Clean or replace the shorted socket.

Systems

• Gottlieb System 3

Source: Repairing Gottlieb System 3 Electronic Pinball Games from 1989 to 1996, Clay Harrell (pinrepair.com)