You might check the barrel nut alignment if you haven't already. If the barrel nut is touching the gas tube, it may not be going into the gas key straight, and causing leakage and/or binding at the key. If that is the case, you should replace both the tube and the key and it's screws. This is an often overlooked point of the gas system. Some AR builders do a great job up to this point, then it seems like they just slap this part in without a thought. Once the rifle has fired a few rounds when the gas tube is binding like this, it is too late to just re-install, and must be replaced.
"Democracy is two wolves and one sheep voting on what to have for dinner.
Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin