The center pop bumper stops working, and at the same moment a few other switches that share its switch row (the 6th pin, a lit-letter switch, a rebound) stop registering. It comes and goes, and cleaning and gapping those switches does not cure it.

Several playfield switches drop out together: worn insulation shorting the start button

Cause: The fish paper insulation on the cabinet start button switch had worn through and was grounding the switch, upsetting the whole row.

Checks

• Use the schematic to find every switch on the affected row or column, including cabinet switches such as the start button, not just the playfield ones.
• Inspect the insulation on the start button switch stack and check it for shorts to the bracket or ground.

Fix

• Replace the damaged fish paper insulation on the start button switch so it can no longer short.

Systems

• Bally MPU AS-2518-35

Games

• Strikes and Spares

Related tips

• One switch or a string of switches never registers
• Pop bumper fires on its own when a flipper is used: replace its switch capacitor
• Pop bumper and both outkicker switches dead: bare wire touching the coin door
• Hitting switches in one matrix row tilts the game: bad capacitor on the tilt bob
• Flipping scores 10 points: a rebound switch squeezed closed by new rubber
• Switch reads stuck closed, or a pop bumper chatters
• Tilt doesn't work and closing it fires a pop bumper: dirty slam tilt switch
• Pop bumper or slingshot fires by itself; random tilts

Source: Pinball Repair help group!! (Facebook group)