As an owner of a 7.5" upper AR .
http://img.villagephotos.com/p/2005-2/955907/Micro-Car.jpg
I had the exact same problem.
I tried everything from the pigtail gas tube, to the rubber spring insert, to the D-Fender thing. None of them worked.
One day a kind Canadian gave me a "tuned" extractor spring and extractor (basically a heavier spring and extractor set). Problem solved immediately!!!!!
Forget the buffer. Shoot several mags with only one bullet in them and if the bolt locks back after each shot, then its not a gas issue.
The extractor is simply letting go of the rim of the shell. Look at the shells and you will see a heavy mark where the extractor grabbed the shell and then ripped off.
If you can't get one of these items, try this...
I took some Bakelite or mabye an old credit card and cut little "D" shapes that can go under the extractor and around the spring.
Don't hinder the spring's operation, limit the extractor's movement to almost nothing. Remember that it still has to move in order to grasp the shell, but don't let it let go!
BISHOP