You need to first figure out if the flipper is sticking up for a mechanical or electrical reason.

Take off the glass and play until you see the problem. Then push on the flipper. If it drops easily then it's just sticky. You need to service the mechanism. For example the coil sleeve might be cracked, and causing friction.

If it resists, then it is still energized. The game either thinks the button is still pressed in, or some other electrical gremlin makes is stay energized. A loose wire of bad switch might do this, which would explain why it drops on its own later with other vibration.

An electrical problem is far less likely than a simple mechanical issue. Once I verified it was a mechanical problem the first thing I would do is inspect the mech top and bottom for obviously broken stuff, and the next thing I'd do is remove and examine the coil sleeve.

From what I've read, the coil stops on these are kinda flimsy and could very well be the culprit behind the problem here. Perhaps try swapping coil stops with a good flipper and see if the problem moves.

Your first statement (figuring out if it is mechanical or electrical) is 100% correct. However diagnosing it like that is subjective. The better way to do it is when the flipper sticks, power off the game. If it goes down, electrical. If it stays up, mechanical. Just easier than removing the glass, and you can't influence the outcome.

So that comes down to the diag. If it goes down when you shut the game off above, it is electrical. So report back on that result, and we will help direct you.

First thing in electrical is the flipper switch. Getting stuck in? Adjusted too close?

First thing mechanical is interference. Is there something just hitting the flipper mech. A manual or wire or something just in the way.

Fair, that is a better test!

Alignment of the coil to the post that runs through it connected to the flipper looks..crooked. Does it move freely? Also - is that coil stop just dirty or worn? That angle looks hard to tell

Try moving the flipper by hand. Does it feel rough when you move it?

Edit: Never Mind

The coil stop is really worn yes so change it. The loose metal piece on the opposite side of the coil you mentioned should be fastened properly too to avoid binding of the plunger.

Coil stops are a well known and notorious Stern issue and have been for several years now. Replace them with Pinball Life ones and away you go. Currently on special so might be a good time to grab a few spares as these are a heavy wear item and will require replacement a lot more often than most other parts in the machine.

https://www.pinballlife.com/flipper-coil-stop-for-sternsega.html
stop (resized).png

Remember. The nipple on the coil stop should be affixed to the coil stop. It’s all one piece. It shouldn’t move at all.

On new sterns, if the flipper is sticking at all, it’s 99% the coil stop.