Magazine issue could be the problem. A few years ago I bought a Bushmaster Varminter did the same thing with quality ammo. Seems like the issue was a leak in the bolt carrier key. Sent the upper back to them and they fixed it no charge...I wasn't very happy to say the least.
Since I got it back I run the entire bolt carrier with a generous dose of Mobil One synthentic oil. Haven't had a problem and the gun is very accurate.