I have not seen anyone mention the following. Ask your gunsmith to remove the belly plate and inspect the tumblers and sears. The belly plate can be removed without disassembling the action. Cycle the gun and watch the engagement of the sears. When the barrels are fully opened see how much clearance exists between the sears and tumblers when the tumblers are fully reward. When the sears snap into their position behind the bent, the tumblers should travel forward no more than 3 to 4 thousandths. The sears need only minimal clearance.
If the tumblers creep forward 10 thousandths or more before engagement they will lose impetus. This most probably means someone has shortened the sears or much worse, filed on the bents. This called "under throwing." This means you need new sears unless it's he "worse" case.
You have gotten rather good advice from the board but the gunsmith needs to hear it if he hasn't already. I have no idea about his skills and pass no judgement. The respondents' advice has been to start simple before getting complicated which is SOP. My advice is simple....before it gets complicated.
Good luck
