I've been chasing this problem down for a long time, but I still can't seem to get them working properly. Most recently, while playing a game, the right flippers stopped working. I kept the flipper button held down, went to switch test in the diagnostics, and the switches were not showing up on the switch test. Tried hitting the flipper button a few more times, still nothing coming up on the switch test. Finally they started working again, and would not act up! Here's what I've done so far:

-All EOS and flipper switch wires test good from switches to connector for Fliptronic board
-Installed new flipper switches
-Replaced EOS and flipper switch connector pins and housings on Fliptronic board
-Re-flowed solder to EOS and flipper switch header pins on Fliptronics board
-EOS switches were replaced when rebuilding flippers, all test good with meter

After trying all this stuff, I'm still getting occasional intermittence from both right flippers and the left flipper--the strength of the flippers seems to fluctuate a bit too. They typically will start acting up within 1-3 games.

Any ideas?
Thanks!

Lots of good info there, wayout440! Thanks! I'd like to try bench-testing the Fliptronics board, anyone had experience doing that? It doesn't look too difficult, but then again, I've never done it before. Can anyone point me to more info regarding bench testing boards like this?

Would I be better off simply replacing the LM339's? The ones on the board appear to be the original parts. Not that I'm one for 'shotgun' parts replacement, but I figure they're cheap to buy and will probably be due for replacement sooner or later.

Thanks for your responses!

Wayout: I'll go ahead and change out the LM339's, and socket them while I'm at it. Hopefully that will fix it, but if it doesn't--like you said, it will at least eliminate one more potential culprit.

Geocab: good call, I hadn't thought to check the ribbon connector. It seems tight, but I'll test it to make sure it's reading continuity. I may just swap it out with a new one to eliminate it as a problem as well.

If I make any progress, I'll report back!