This sounds like a case where the gun works OK when operated slowly. But when operated rapidly, the cylinder stop does not come up quickly enough to stop the cylinder at the right point. The reason could be a defective or broken stop, but probably is either a weak spring or dirt/grease in the mechanism. I would take off the grips and hose the gun down inside with a gun cleaner, let it drain and see what happens. You should not have to remove the side plate to do that. Then if that works, put a few drops of light gun oil in front of and behind the trigger, and a few drops in front of the hammer and it should be good to go.