Just for the hell of it, I'd like to see you replace the cap on the expansion tank and see if the pressure in the upper hose goes away.

If the cap has pressure issues it can create havoc for the system.

One other item:

Have you had the cooling system opened up for any reason in the last few weeks? Is it possible that there is excess air in the cylinder head? On mine, when changing coolant, the temperature went right over 100°C. upon first start. Had to fill the head via the upper hose to get the temperature down. There is a lot of places to trap air in this system. If you did have the system open, the air in the heater core won't be released until the system is run with the temperature dial on full hot for 10 minutes or so.