As a damn good M/B tech, I think you have a combustion leak into the system. Maybe head gasket, maybe cracked head. I would monitor the cooling system pressure closely. Release the pressure off of a hot engine and hook up a gauge to it. Rev the snot out of it for a bit (2000 to 4000 RPM) and observe if the pressure rises. A hot engine wouldn't have much thermal expansion so a rise in pressure on a hot engine indicate a combustion leak. That means the head comes off.