Had the same problem with my 424XXXX Springfeild trigger assembly. I would fire a couple of rounds and the same thing would happen, it is kind of embarrassing standing on the firing line when this happens. Well anyways I checked the tab and the lock to see if they were worn. But were fine, and the fit into the stock was tight with no receiver movement. And tried it in two different rifles to the same effect. Well my guard was milled not stamped, and checked it out compared to other milled guards and found that this guard dident curve as much as the others. My solution was to squeeze the guard in my hands to close the gap slightly between the front of the guard and the lock( with the guard open not locked) and test fired. Problem solved, and have not had a problem since. That was a few years ago. Give that a shot if you have no signs of worn parts on you trigger assembly.