I am trying to diagnose a cooling problem with my M104 CIS motor. I've installed a new water pump, thermostat with ball valve is pointed between 9 and 10 o'clock position as pictured in the 124 manuals, bled the water system, but I still get gradual overheating problems on the freeway. The needle goes over the 100C mark cruising at 70mph and backs down if I slow down.

I notice my upper radiator hose has a lot of pressure behind it and I cannot squeeze it after a drive. The hose is not stiff/caked with stuff lined on the inside, I can squeeze it just fine if I release the pressure via coolant tank cap. I am also hearing sounds of quiet/slow pressure leaking from the coolant tank cap. Is this sign of overpressure? Do I have combustion pressure leaking into my cooling system? Is there a way I can confirm/test this to see if I really need to pull the head for a head gasket replacement?

I am not seeing an oil in my coolant.