Thanks, Good advice.
I checked the lock screw before and again just now. It seems fine. I encountered the long screw binding with the hammer when I first built the kit. Addressed it at the time by filing the screw a little shorter. And I always check it when reassembling.
I just got back from the range and it did fine for 15 or 16 shots.
Since it is intermittent I'm thinking its not a long or over tight screw. IMO those would make it bind consistently. So, I'm thinking something in the internal mechanism. I'd like to take the lock out when it happens but so far it only happens when I've got a barrel stuffed with powder and ball. Maybe, I can stick some leather over the nipple and shoot it a bunch without powder and ball and see if it happens. If it does, maybe I can take it apart and see what's catching the hammer.