Problem with a MkIIIB bolt jumping its track

[ForgottenWeapons]

I just got a pretty nice MkIIIB Ross, but the bolt is giving me some trouble and I wonder if anyone else has experience with the problem. When fully opened, the bolt head often (about 50% of the time) jumps off its guide rails and starts to rotate. I can usually get it back down and horizontal, and when I can it will close just fine. But the rear sight bridge interferes with getting to the bolt head in this position, and sometimes it rotates significantly and gets thoroughly jammed up. Once I managed (unintentionally) to get the bolt head fully rotated into the locked position with the bolt in the fully open position. I disassembled the rear end of the bolt to remove spring tension on the bolt head in order to unjam the bolt.

Is this a problem with a well known cause and/or solution? FWIW, the bolt has not been safety riveted (although I don't expect that would have any bearing on this issue).

Attachment 44699

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