help with m1 carbine rear sight needed

[Spermgewehr]

Dusted off my Standard carbine today and took her to the range. I shot a few groups at 50 yds but soon discovered my later shots were printing higher than the first. Its a standard rearsenaled m1 with the bayo lug and windage adjustable rear sight. I put the sight back to 100, fired, and found that the peep moved back to 150. I put it back to 100 and again it moved up. This seems to be a new problem, or at least one I didn't notice when I was shooting steel silhouettes at about 70 yds last time. Anybody got any easy fixes for this?

[Junk Yard Dog]

You can replace the sight , or examine it closely to see how it's being held together. Sounds as if it's gotten worn to the point were the detent that holds the slide in place is loose. I don't know if it can be tightened up, that will have to wait until I am home and can examine one of mine.