Diode or EOS switch is my guess

This is not a diode issue. You end of stroke switch is adjusted incorrectly and engages too soon. It needs to be adjusted so the switch triggers later in the stroke. Lift the Playfield and move the flipper while watching and see how the mech closes the leaf switch blades then bend them accordingly

Perhaps your eos switch is dirty or not making good contact. You could try sanding the part between the blades to test. One way or the other, this has to be caused by EOS. And if you were to remove EOS, the flipper bat would just drop cold in your test.

The theory on how stern flippers work. There's a single pulse then hold power is applied until the flipper button is released. At any point while the flipper button is held if the end of stroke switch registers open, the cpu will send another high power pulse. This is seen when a ball flies into the raised flipper. The EOS should be close enough to the end of the stroke that if another high power pulse is sent that the player cannot notice the event happening. If the EOS is catching late then the cpu sends the second high powered pulse while the bat is slightly lowered and now the player sees the flipper do a mini flip.

There is no separate hold power circuit on sterns. So if the coil works at all then the hardware behind the flipper mech is working. The only issue that can cause this on a stern is a faulty or mis adjusted end of stroke switch.

Therefore, I would look into things like EOS wires making good contact, EOS adjustment. Or maybe just install a new EOS on the thing to be sure.

You really only have 3 things left to look at...the the flipper button switch (doubtful), coil (doubtful) or a logic/software issue.

Stern flipper coils use a single winding. When the flipper button is pressed, the machine fires the flipper coil for a long flip pulse (30 milliseconds or so) and when it detects the EOS switch open, the machine immediately follows with short pulses (maybe 2 milliseconds every 20 milliseconds) to keep the flipper active until the flipper button is released without applying full power to the flipper coil so the flipper coil doesn't burn up.

When Stern released the last ST update (1.5), something they changed affected the flipper code and occasionally the upper right flipper does this where it didn't before.

Things I would try one at a time in this order: a factory reset, download a fresh copy and reload the the last software revision, swap driver boards from another game, and swap CPU from another game. I know the last two are a PITA, but it may point to where the problem lies.

Reviving an old topic - I had an Iron Man with the same problem - pulled the driver board and swapped - it worked. So I checked out the board - Q15 and D15 had been swapped, so I pulled both and replaced. Also put in a wire jumper from D15 to that tiny surface mount cap since the trace looked kind of iffy. Works like a champ now.

Scott