Two items come to mind. First is the trigger plate on which the triggers are mounted is moving around allowing the trigger to rest on the sear. When you fire the gun there is just enough movement to partially disengage the sear from the hammer. When the hammers are cocked is there any play in the triggers, there should be a little movement. if not try putting a shim cut from a playing card under the trigger bar and see if this helps. A lot of older guns have had the trigger bar removed and replaced a number of time and this can compress the wood to a point that causes problems.
The other area to checks is there the sear arms pass thru the stock (usually thru a hole running horizontally. Again over the years there can be some wood swelling or the wood below the lock plates can get depressed allowing the sear to ride on the triggers.
In either case there should be a slight play or movement in the trigger.
Hope this helps...
WBLDon
