Sounds like a seasonal framing movement problem. Adding more tape/mud is not going to work, as the movement of the house is much stronger than the tape/mud.
You might try waiting until the gap is at the widest and filling with a flexible caulk. That will allow for movement without seperating. However, that will probably cause a bump when the wall contracts.
The only complete fix is to remove the sheetrock, find the reason the framing is moving, and repair that.