it's been a while since i've seen one do that, but IIRC i've seen it happen with with worn FCG (trigger group) parts. the FCG on the Plum Crazy lowers is plastic. and while i don't have one, it MAY be possible for the plastic FCG parts to be out of spec, or have the engagement surfaces wear prematurely for some reason.
tere could be other causes, and i'm not saying that plastic is bad for everything. i suspect that a new FCG, either metal or plastic would resolve the problem... however it's not the only possible cause for the problem, just seems like a likely one.
sounds like Guido will take care of you when you take it in.