I'm having this exact same issue and it's infuriating. I've replaced the flipper boards (both original and PinLED), traced the wiring throughout the whole machine and tested every lead.

Do you have a shaker motor? Mine is powered through the flipper board and I think the aftermarket connections might be the problem.

I had the problem with the original boards and switched my lower flipper board (front of the cabinet) to the upgraded model and had the same issue. When I switched the boards the problem persisted.

Next, I disconnected one plug at a time from the flipper boards and did a switch test for each. The problem stopped when I disconnected the lower flipper board data plug. (I don't have schematics handy at the moment - the plug closest to the front of the cabinet; not the power input connection.)

I noticed that in the schematics and figured that's the most likely place for a connection. I'll work up the courage to solder in some new transistors, but it doesn't explain how several different boards give the same result. Grantopia mentioned flipper coil diodes, which I hadn't considered.

I was just going to post! I ordered some new transistors, replaced the Q5 and the Q7 on the original board and the switch issue is gone. Unfortunately, the flipper power is a bit weaker and it's harder to make the ramps.

The flipper power is great with the PinLED board, but while the Q7 is replaceable, the Q5 is a micro component and can't be removed by hand. So to get a working game with good flipper action, I'd probably need to buy another new board.

Unless someone can guide me to more adjustments on the original board to boost the flipper power. Maybe a lower-power resistor or a higher power capacitor would do it?

Well, dang it, Gran. My incline was set at 6.8 degrees. I lowered it to 6.0 and it's way better now. Plus I can make the side ramp.

Why is my first instinct always the most complicated and expensive?