I have a flat top .44 special that was doing the same thing. I replaced the loading gate spring and it has been working fine since. The cause is, I believe, that the little hook end of the gate spring that is supposed to lock into the groove around the trigger pin is just a little too wide to properly seat in the pins groove allowing it to slide forward under recoil. A new spring fixed mine but possibly grinding that area just a little narrower so it can go into the pins groove would be just as good a fix.
That said, mine would lock up completely,The loading gate could not be opened and the hammer could not be cocked or could I remove the cylinder. I had to remove the grip frame to free it up so I could get the cylinder out to even unload it. It happened on the 13th shot from the factory. I put everything back in place and it locked up again on the 8th or 9th shot. After replacing the spring It has been fired about 80 shots without issue. More testing will be done soon.