Odd stuck mirror problem on Canon F-1n

Because we don't have enough threads on FD gear already, here's another one. I've been having a rather strange problem with my Canon F-1n (2nd, mechanical version, not the New F-1). Sometimes, the mirror gets stuck after an exposure and doesn't return to its viewing position. The shutter closes, but the mirror stays up. The only way to get the mirror down is to dismount the lens. The odd thing is, this only happens with my FD 50mm f1.4, and only at 1/125th shutter speed. All other speeds are fine, and so are other lenses at 1/125th. I don't have another camera body right now to test the 50mm, but I know it works flawlessly and doesn't have any bent or crooked parts that I can see.
I tried cocking the shutter prior to mounting (I always do that anyway), but that doesn't fix the problem. Also, the mirror doesn't get hung every time, only about once in five times.

So, what's happening there? There are small scratches on the side of the mirror, indicating that it rubs against the rear side of the lens. But why, and how, and and and... I'm confused

It should be impossible for any part of the lens to contact the mirror. The only explanation I can think of is that the rear cell might be partly unscrewed.

That would place it too far rearward and possibly it might then contact the swinging mirror. If this is the case, then by screwing the rear cell fully into position, it should move forward and clear the mirror as it is supposed to.

If you can get the mirror to hang, see if it will release by pushing the curtain gently to the left. If it releases, then the curtain tension needs to be adjusted.
BTW, no film in the camera =)

Have you used the cameras' "mirror lock-up" mechanism? Does the mirror easily go up and easily go down when engaged? Any binding or rough action?

Jim B.