My guess is the lever. On my Gov. Model, there is a small lip on top of the lever just behind the lip on the grooved thumb surface. That lip rides inside the slide and retains the lever. The lever is removed by holding slide so the small notch, the one behind the slide hold open notch, allows to the lip to pass through the notch.
The plunger might provide enough pressure to keep the the lever in place during operation when the notch passes the lip, but if it is walking out gradually, it my guess would be the lip is worn or missing.
+1
I'm thinking the top of the stop is worn enough to let it slip under.
Mic (or use calipers if you have them) the thickness (from top to bottom) on the part that is retained by the slide. Check it against another you have and see if there is a difference.
Remember that the slide has to be retracted to a certain spot so the stop can go through the notch and be retained by the slide.....regardless of the spring and plunger.
I'm pushing on one right now and it won't move.
Remember, once the slide stop is in its proper place, you can't (or shouldn't be able to) push it to the left...unless it is lined up with the take-down notch on the slide. If the stop comes out in any other position, it is worn on the top edge and should be replaced.
Put the old one in properly and with the slide in battery, it should not move to the left when pressed. Theoretically, it shouldn't move when firing because it would have to jump through the notch......but I have seen it happen and lock up a slide in action.
I am definitely interested in the cause/solution in this one.
**As a side note, the Colt I have in my hand does not have a dimple on the factory slide stop either.