Quoted from Wolfmarsh:Hey Bas, serial LEDs flicker like that when there is a power problem or a flaky connection somewhere.
Ground and 5V are super important. I would recommend you run some extra 5V and Ground lines directly from your power supply to the LED chains, about every 10-15 LEDs. If you are having floating ground issues that will make a massive improvement. Just connect them in to your existing chain wiring.
Doing this could also create conductive loops that are susceptible to induction due to magnetic fields acting on those loops. So maybe just supply power near the middle of the chains and not at the ends? Took JJP a long time to solve this on WOZ, if ever.
John will agree with me: PINBALL IS HARD ( - :