If you have a hammer spur (extension), take if off and see if that helps. I occasionally got misfires with mine until I did that.... although they were not predictable like what you describe.
I honestly don't know. One thing, people occasionally shorten the mainsprings to get a lighter trigger. If that appears to be the case, then I would suggest having an extra spring on hand.
Pretty odd.... good luck.
If the removal of the hammer spur stops misfires I wonder if the hammer springs eventually weaken? Is it a good idea to replace the hammer springs occasionally? Or should you shim the springs with small washers?
