Something I read, but can't find right now...load your mags with three rounds, and see if the slide locks open. Then 4, and then 5, and so on...to see if there is a point where the problems begin with a certain number of rounds in the magazines. Also, note if this occurs with ALL magazines, or only one of them.

Also, I assume you bought the gun new? If not, do you know the round count? Seems unlikely, but maybe the recoil spring is the culprit.