Just a thought, but does the hammer show any sign of dragging on the frame? Could be some interference that is slowing the hammer strike. Perhaps the mainspring strut even?? Try cocking, then slowly lower the hammer paying particular attention to any change in the feel, a bump or lessening of the force required to hold it from falling free. Do it very slowly.
A question, if you mark the chambers, 1 thru 6, does the misfire repeat on a particular group of chambers? This has become and interesting issue, but then it ain’t my gun