I'll quess the fuse clip on the Flipper board has a cold solder and needs to be reflowed

This might be the problem.

http://arcarc.xmission.com/Pinball/Stern%20Service%20Bulletins/sb/sb54.pdf

If you can get it in the "failed" state, squeeze the EOS switch contacts closed tightly while holding the flipper button in. If that makes it fire, the EOS switch is dirty or bad.

Another thing I've encountered is when the gold plating on the flipper button switch gets worn away, the switch becomes intermittent. New switch at that point, but make sure you use a good plated switch; the old style (tungsten?) Switches will seem to work but become intermittent soon.

Remember, the TYFFASI flipper boards have LED indicators on them when the cab switches are pressed. That may help eliminate debugging making sure they're lighting when you press the button, at least you'll know the board is seeing the cab switch close. I'd also clip an alligator clip on the flipper coil while doing that, make sure it is getting the right voltage.

As I think might have been noted, did you look at fuse clips on the flipper board? This maybe some intermittency you are observing. Possibly some cold solder joints elsewhere?

The flipper board is not terribly dense; pretty easy to navigate around with help of the schematic. Easy to find the left-flipper circuit and associated components.

dup

mine was the eos switch. i changed cpu boards, flipper boards, coils, and then finally the eos. turns out it was the eos the entire time.

Jumper across the EOS switch with some alligator clips and test it, if it works every time replace the EOS switch.

This is your answer faqawi