Curly, there a couple of things that can help. It is possible that your frizzen spring is not applying enough force on the frizzen to prevent it from snapping back. to correct this problem, remove the frizzen spring, heat it red, and spread the arms of the spring just a bit. Then polish it bright, heat it bright red and quench it in oil. Again, re-polish it, being aware that it is glass hard now and will break if you flex the arms at all, or even drop it onto concrete. Draw the temper ... I do this on a copper plate that is roughly 3" square x 1/4" thick. I drill a small hole part way through to accommodate the tit on the back of the spring, lay it flat, and heat the copper from the bottom with a propane torch. I hold the copper plate in my machinist's vise by two corners. I put a drop of motor oil on the plate as a heat indicator...when the oil starts to smoke, you're getting close to the heat you need. Watch the colours slowly change on the brightly polished spring. First it'll turn yellow, then a darker straw colour, then purple...slow down with the heat - back it away, and watch the spring turn dark blue, then pale blue. If you haven't overheated the plate, you can just leave the spring on the plate til it cools, but you don't want to anneal the spring much further, so you can remove the spring with tweezers, and place it on a wooden board to cool. When you can pick it up with bare fingers, you can install it on the lock and try it for action with the frizzen. This should take care of the re-bounding frizzen problem.
Another thing that might help is to file/grind some metal off the frizzen's stop...the part of the frizzen that contacts the frizzen string and stops the frizzen from rotating further. This will allow the frizzen to rotate just a bit further forward, allowing the spring to apply more pressure on the foot of the frizzen, ie: that part of the frizzen that rides on the spring and keeps it closed on the pan.
Do not file metal off your mainspring...it is supposed to be stiff and strong.
