I had the same problem tonight. First time out at the range with my new Sub. At first I thought there was a feed problem, but no, the gun performed flawlessly but I had to hold the safety in the "fire" position with my thumb while shooting. This is freaking annoying and I don't think that wiggling it back and forth is going to solve it...will try though.
Please post up if any of you figure this out or find that the problem solves itself.