Hmm that's a tough one from the description. I would normally say its may be the disconnector spring got interchanged with the bolt catch spring, but since it worked fine for the first 300 rounds, that makes me scratch my head.
I'd check the gas rings on the bolt to see if they're lined up. If they're fine I'd check the gas tube to see if its clogged, then check the gas hole for obstructions. You can remove the gas tube by knocking out the roll pin in the sight tower, pulling the tube back into the upper, rotate it to the side then pushing it forward. If you need to check the gas hole, you can remove the sight tower by knocking out the two pins and tap the tower forward. You can clean the hole by taking a drill bit (approximately the same size as the hole) and insert it into the hole and roll it around with your fingers.
This should take care of the problem, I believe that the bolt is coming back far enough to extract and eject, but not reset the hammer.
This generally doesn't happen, but some types of ammo burns dirtier than others, and you may have had a bad batch.
Give that list a try and if its still giving you problems give us a call.