Quoted from gdonovan:1) So I should make sure the outhole switch wiring is as far as possible/practical from the flipper wiring.
2) would caps on the EOS switch help?
Its rare that it does it for what it is worth, maybe once a game tops. Flash Gordon is far more prone to it than my homebrew Seawitch.
I have a Vector that has pop bumper firing occasionally. It was much worse. To improve the situation, I separated the harness as much as possible to keep signal lines and 'power' lines apart. I added caps to the EOS switches on all 4 flippers and I added caps to the flipper buttons. All that said, it improved, but did not go away 100%. As Quench has said, it is inherent in this design. Without shield cabling and proper routing, maybe some RF chokes, etc....it will never be perfect.
I never put my scope on the switch matrix, but definitely noise spikes happen. For me, it is always the pop bumper and everyone once in a while the ramp timer starts because the MPU thinks the ramp switch was triggered.