I had the exact problem on my first AR (a Bushmaster 16" preban). I concur it is a GAS LEAK. Probably from the bolt carrier/carrier key.
Mine started doing it after I shot my first 1500 rds of Wolf Ammo. I started reading about it and everyone was bad-mouthing the Wolf Ammo saying how it is dirty shooting, gums up the gas system, gums up the chamber with laquer, making rounds stick, delaying the extraction, short cycling, not picking up the next round,...blah..blah...blah. (sound familiar??)
I was pissed at myself for buying cheap ammo in order to shoot more and increase proficency. (not a high point for me) and I went 6 months to a year without shooting it (work/family does get in the way!)
Did all the cleaning/ oiling (but not over-oiling), mag swaps, ammo swaps. Nothing.
Broke the rifle down TWICE. Meaning down to the individual pieces. (ie gas block off the barrel cleaning with pipe cleaners). Took a chamber brush and my Makita to the chamber. Nothing!
Looking at the carrier and carrier key I could see that the Allen head screws on the top were staked in as they should be, and there is NO WAY they could have backed out........WRONG!!!
I put an Allen wrench to them and they spun freely! So I removed the key and checked the mating surfaces of the carrier and key and found streaks of carbon where gas had been escaping. Gas escapes, delays unlock, bullet exits barrel, gas pressures go down=short cycle!
See this manual posted here (http://www.ar15.com/content/manuals/TM9-1005-319-23.pdf) and look on page 265 for a good shot of the Carrier and Carrier key.
I cleaned the surfaces and went over them with some steel wool just to make sure. Put a fine thin (barely visible) coating of Moly Grease on the surfaces, put the key back on, and torqued the shit out of the Allen screws.
That was 4 years ago (and 5 AR builds ago which taught me alot too) and my Bushmaster runs like a sewing machine!!! It never hiccups and it'll eat anything!!
The AR is a great rifle and will function as advertised. If it doesn't, the problem is easily fixed due to the modular design. Just find a manual and run through a troubleshooting checklist. There was a discussion on another post about whether the AR was "built" or "assembled"
by the homebuilder. I think the beauty of it is that its assembled and can be serviced by the home user easily.
Hope this helps!
SnowFAL
Hope this helps!!