Thanks, guys. After further inspection, the magazines are indeed suspect. The springs seem weaker than they should be (completely subjective, of course) and the followers are lightly worn. As they're high caps, I don't really want to replace them. I'll get some Wolff springs and new followers and see how it goes. The original factory grips are wood and bind a bit on the mag, making it not drop out freely when empty. I may get a set of black plastic grips, too (or Houge rubber ones?), and see if that helps.
As an aside, this is the first pistol I've ever owned, and I don't want to get rid of it - but I don't think I'll buy another Beretta anytime soon. Despite the good accuracy and nice feel, I'm really disappointed in it's reliability. Only four or five hundred rounds and the bloody thing is malfunctioning. Aren't Beretta's supposed to be great, reliable pistols? A few of my friends and one of my brothers have them too, and all have had some sort of problem. Just disappointed, and griping...