I'm a guessing, without being there, that you're bending/flexing the action by over-tightening one of the screws. If you've got a three-screw set up, which I don't know since I don't work with mausers, the middle screw would be the likely culprit. If you've got a two-screw action then I think you need to see where you're tightening the action and flexing it. If the barreled action fits down into the stock all the way, such that tightening the screws does not draw the action or barrel any further into the stock -- then you should have a good setup. If you can't get the barreled action seated into the stock without the screws drawing it up further then you might have to relieve the stock some more to eliminate the flexing.
Just my thoughts,
SS'