^^^^^ THIS.

I, personally, do NOT recommend the Kahr board as a primary (long term) solution. It's great to keep a game running on location or at a show but it's not a long term solution. I only have one that I use to keep any game that exhibits this problem running at the Northwest Pinball and Arcade Show. Once the show is over the problem is isolated, fixed and the board removed.

The simplest (and most expensive) option is to just get a new board. Fix, sell or give away the old board. Don't throw it away. It's fixable.

The next most expensive option is to send the board off to a professional repair service (as suggested).

The cheapest option is to fix it yourself - I suspect that the big capacitors servicing the +5V and +12VR are failing or possibly have a solder joint problem. What you described sounds like AC ripple.

You're local to me and I'd be happy to come over and help you fix the problem. If you're interested send a PM.