The MPU boots sometimes and not others, or faults come and go when the board is touched.

Intermittent MPU faults from old closed-frame chip sockets

Cause: Brown or black closed-frame sockets (you cannot see the board under them), including ones stamped SCANBE or RS, lose contact with age.

Checks

• Look at the ROM, PIA and RAM sockets; any closed-frame brown or black socket is suspect.

Fix

• Replace every closed-frame socket with a machine-pin socket.
• As a stop-gap while diagnosing, push a machine-pin socket into the old socket and plug the chip into that.

Systems

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

Related tips

• Intermittent or dead MPU from old closed-frame sockets
• Reseat the MPU chips before anything else
• Get the MPU booting before you change any ROM jumpers
• Strip the MPU to three chips to get the first flicker
• Power and ground pins of the main MPU chips
• Game will not boot, or MPU damaged, after swapping -35 and -133 boards
• Double-check bench leads: a +12 V to +5 V short kills the MPU chips
• Stern M-200 occasionally will not boot and the MPU reset button does nothing

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