Solved.
It was not the spring.
My sons Husky does exactly the same thing. It tended to be the third round where occasionally the rear of the case did not pop up to catch the bolt. OAL was not an issue because at this point the projectile did not touch anything.
The top of the magazine box at the rear had a sharp 90 degree edge,and the back of the case appeared to be catching just slightly on this. (Cigar for BillyDixon!) I judiciously took just the edge off and polished the top half and now you simply cannot make a case stick if you try. There was also a burr in the ejector slot which had some brassing on it so I removed and polished that too.I guess it had never been particularly noticable in the field as its seldom I would fire a third shot and possibly recoil would pop the round into place anyhow.
Either way good to know its more reliable now as a mauser should be.
Cheers.
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