Holy cr@p what a blaster! I'm thinking recoil spring/guide too. Not that you haven't tried
this, but I would manually cycle it unloaded a bunch. I mean hundreds of times. It
gives these new parts some "time" to get to know one another. Strip it, blast it with your
favorite cleaner, lightly lube, put it back together and test fire.
...if this doesn't work, save yourself the frustration and give it to me.