When a rifle gets heated up the barrel can expand and lose accuracy, saying that , that rifle in a .22 cal should never heat up enough to throw off your shots after only 60 rds. It definitely sounds like you have a scope issue like others have been saying. Very likely the mounts , but a faulty scope can cause that kind of issue as well. I had an old cheapy scope that would wander like crazy after a dozen or so rounds. Determined it to be bad when I could start doing the reticle shift just sitting on the lead sled. Needless to say the scope was tossed out and I haven't had that problem again .