I think we are overlooking something very simple here.
Sounds like possibly a gas key has started to loosen. Clean your bolt carrier/key well with some BrakeCleen or similar and allow it to dry. Take one drop of solvent and carefully place said drop in the gas key and allow it to run into the bolt carrier. Examine the joint of the gas key/bolt carrier carefully. If you see ANY evidence (wet) of leaking at the joint then you must have the key lapped and reinstalled.
You can check your gas block the same way.
With the bolt sticking @3/4 it sounds as if it is short-stroking.

One other thing to check-Make sure that the hole on the upper stock screw is not plugged/blocked in any way. If it is you will get a short-stroke because of the retardation of the buffer traveling. All it takes is slowing it down by milliseconds to throw the entire timing cycle off...

If you are still stumped shoot me a PM.
Good info Quib.......