I don't have any experience on that EXACT weapon, but I have ALOT of time behind an M-16 A2. What you are describing actually happened alot. It was usually the magazine. It sounds stupid, but the way the magazine is held in is pretty basic and not very secure, so if the magazine would shift forward too much, or be going in with pressure in one direction or the other, etc. It would cause the bolt to keep getting caught, double fire, etc. We realized it happened most of the time because of resting the rifle on or against the magazine while in the prone supported position, or using it as a handle/brace in any other firing position.
Dirt and grime only make the problem worse. One day, it was about 50 degrees outside, wet, muddy, etc. We were running from barrier, to car, to building, firing the whole way, covering each other while we moved, etc.
By the end, I was having to drop my magazine, clear the chamber, and reload literally after every other round.