I've just checked a rifle that had a similar problem - bolthead jumping the track. Turned out that the rifle (a 1942 Dispersal, rebuilt as a complete horrid bitsa by some genius) seemed to have a perfectly decent receiver, but it had a mismatched bolt where someone had further ground the locking faces. Headspace was off the scale, and the bolt had considerable lateral and vertical movement. A change of bolt gave a nice tight action and solved the problems instantly.

An old armourer & gunsmith mentioned that "worn or distorted bolt rails" were extremely rare; their shop method involved a quick application of a cold chisel down through the magazine well to persuade the right hand side to move over a bit....