I agree with Justus. it definitely sounds like a powertube issue.
You mentioned that you changed carriers and such, but you never mentioned removing the shims. Too many shims will cause your problem. A worn sear will also cause the problem.
Always remove the shims when tuning and testing a level 10 bolt for leaks or force. I wouldn't even put them back in if they were there. Then use the largest carrier that does not produce a leak using the same white powertube oring in each carrier.
Except for the Automag in front, its usually the man behind the equipment that counts.