Is this common on the K31?
Not that I've really ever heard of, I know mine is pretty consistant and doesnt do that.Possible some of the screws are too loose, or too tight, and when it heats up a little, the barrel or part of he receiver makes partial contact with the stock messing it up.Thats probly what I'd look at first,is the screws If they seem ok, see if the handard is just lightly touching the barrel anywhere, causing the same problem when it gets hot. Might be a small spot in the barrel channed that expanded a little at some point, and is just barely touching, and as it heats up, it presses harder and harder on the barrel.See if a dollar bill will slide under the barrel the whole length, if not, lightly sand the spots its touching until it does, and that should certainly help out.I'm told these can be kinda sensitive to any touching since they were designed to not make contact at the barrel (the other choice is to bed the whole thing.I'm told either works well, as long as the pessure, or lack of, is the same everywhere, our good to go.) Never experianced ayn of this peronally, but this is what I have read an been told, and it seeems to make perfect sense to me, and seems like a likely cause of the way it was acting on you, IMHO.
If I'm way of base here,I'm sure one of the may k31 lovers here will straighten it out though.By the way, REALLY great price for a k31.They have steadily been getting pricier and pricier the last couple years.Good job.They seem to be averaging closer to $175-$200 last I looked.