Ran about 500 through it today. Lots of ftf, I think the spring is too weak, or the cases are hanging up in the mag body, creating a lack of pressure on the rounds above. Most of the time, the bullet wasn't rising up into the feed lips properly, and with a tap the spring would tighten up and then it would feed. Every third or fourth would do this.
Nothing to lose now, I'm going to pull it apart and see if smoothing things out helps.