I'm stumped. I've rebuilt flippers more times than I can count, and for better or worse, haven't come across this issue.

On my recently acquired Twilight Zone, I rebuilt the flippers with new coil stops, sleeves, and plunger/links because I noticed the left flipper wasn't as strong. Starting up the game, everything is great. Flippers are strong and I can make shots no problem.

However, after just a few minutes (it seems this is taking shorter and shorter time to occur), the left primary flipper gets weaker and weaker until it merely flutters. The coil is hot to the touch, as in, holding your fingers on it would burn your hand. The right flipper (which performs just fine), is warm but nothing noteworthy.

The flipper moves smoothly by hand, with proper play. I checked the EOS switch and even used a business card to remove what very little grime was present. Switch levels show the EOS is getting tripped.

I don't get it.

Could you do me a huge favor and explain the "why" behind this? I assume just a cotton swab w/ alcohol will do the trick?

Disregard. Shows I haven't had many Fliptronics II issues! Just read the pinwiki on it. Interesting.

No.

Have you tested the coil diode?

Yes. Resistance is perfect.

Have you looked closely a the solder leads?

Yes. Solid.

Heat damage failure?

No.

Cold solder joints?

No.

Crystalization damage?

No.

Flipped bushing cracked or broken (easy to miss)?

No. Checked this during rebuild. No horizontal play. Broken flipper bushing would cause poor flipper performance initially at startup as well.

If so, the coil may already be damaged and requires replacement.

We're good.

Remember flipper could see a "two stage" function.
High power to flip, low power to hold.
The coil is most likely getting hot on the hold stage.

This was my initial suspicion with the EOS switch, that it wasn't getting tripped during cradling. So last night during my tests I fired the flippers like a 3 year old with no cradling. Same problem.

Fliptronic boards are another leading cause based on game age. Test the board related to the coil function.
What exactly has been electronically tested?

Nothing other than a cursory look at the Ft. board to ensure no obvious damage. People waste tons of time and money shot gunning parts that ultimately did not need to be replaced. That's the premise behind my post here. For whatever reason, my WPC fliptronics games never had this occur, so I never even noticed that section in the pinrepair guildes. I'm going to clean the optos first and hopefully that works. If not, we'll swap boards and see if it's the LM339.

You have already checked most mechanical parts.

Played the heck out of it and so far the Q-tip + cleaning of the optos appears to have done the trick. Will continue to monitor and if the issue returns, at least there are additional board related steps to check.