With one drop target down, a hit on the lower pop bumper makes a different drop target switch register and the bank resets.

A drop target switch registers when the lower pop fires: replace the pop coil

Cause: The lower pop bumper coil itself was the faulty part; vibration from the bumper had already been ruled out.

Checks

• In switch test, close the two switches together and see what extra switch appears.
• Check the switch diodes in that part of the matrix.

Fix

• Replace the lower pop bumper coil.

Systems

• Williams System 6 / 6A

Games

• Gorgar

Related tips

• Hitting a pop bumper resets the drop targets: faulty pop bumper coil
• Pop bumpers do not show in switch test: pull the ring down to test them
• Pop bumper or sling fires but does not score
• Drop targets sometimes don't score: worn horseshoe contacts
• The switch test cannot check pop and sling trigger switches
• Hitting the left pop bumpers tilts the game: failed PIA on the MPU
• Odd behaviour from a short in the tilt switch column
• Switches stuck on: board or playfield?

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