It sounds like the hot gasses venting through the touch hole is gas cutting away at the lock. This condition is usually caused by the one of the two reasons Michael said. If you weld/braze up the lock, eventually the same thing will happen. Since you are not going to move either the lock or the vent hole, you could simply recontour the pan with a grinding point to a nice smooth contour, slightly lowering the bottom of the pan to the level of the bottom of the gas cut or slightly below. This is assuming that there is sufficient metal on the bolster to do this. Make sure that the bolster fits snugly to the side of the barrel to prevent any primer leakage into the lock mortise. Listen to your lock it is trying to tell you what is wrong...the fix is simple.
