I had a similar issue with my Space Invaders. I ended up replacing EVERY capacitor AND diode across every switch in the switch matrix (according to the schematic). The left Pop Bumper would randomly fire when pressing the left or right flipper button. This would happen once in every ten games. Replacing the components in the switch matrix did not solve the problem.

I had an Altek board, and I understand that due to the architecture of the board ( and the clock speed is faster ), "phantom" pop bumper firing would be totally eliminated. I installed the Altek, and there was an improvement. The "phantom" pop bumper firing was not totally eliminated - it still happened once every 20 - 30 games.

These switch matrix issues can be a real pain to figure out.

I would swap the PIAs over and test. This will eliminate the MPU.

You could try swapping out the SD board - CA3081 chip might be firing the pop, but the pop happens when the flippers are pressed.

1N4148 are faster switching diodes. I try to use 1N4004 / 1N4007 in the switch matrix.

Your Skateball was most likely from Germany.