I'm getting DMD message: 'Check Switch 51 Right Trough' when powering on Popeye every time now.

I replaced both the opto trough boards with ones from Great Lakes Modular and even put a plastic Trough Shim in to make sure the balls
keep rolling downhill as they should.

I also was always getting EOS DMD messages on the Left and Right Upper Flippers a lot but they always seem to clear themselves from the
DMD after playing a while (Credit Dot goes away and no reminder next dozen or so games).

I also have a new Rottendog CPU board in there since July 2017 and all lower CPU connectors replaced.

Best way to trouble shoot this issue?

Thanks in advance!

Update: Fixed.
Solution (at least for me)

Empty all 6 balls out of the pin.
Put game in SWITCH LEVEL test.
Roll in each ball thru outhole to watch opto's and what position ball ended up in.

After reloading all 6 balls thru the outhole and watching the switch levels, I couldn't see any issues.

Then the CREDIT DOT and SWITCH ERROR disappeared.

About 10 games, 3 days later, still no credit dot. Go figure.