Bad head gasket will leave the cooling system pressurized cold, so will a cracked head. If the upper radiator hose is hard first think in the morning, suspect a combustion leak.

It will blow "excess" coolant out when there is too much in there. On my cars, "full" is the seam for the top cover of the tank, a couple inches down. More than that will probably over-fill the tank and spill out the overflow.

I'd also pressure test it -- it's possible you have a small leak somewhere that prevents it pressurizing properly, and this will do cause all the noted behaviors. Look for a gouge in the sealing surface in the tank neck, too!

Yes, a clogged rad should make it run too hot under load, but that is exactly what you describe, isn't it? Just running down the highway isn't that much of a load unless you drive with your foot to the floor all the time, but climbing hills is!

Running 95C in traffic is usually a bad visco clutch and resultant poor cooling airflow, but a bad radiator will do the same thing. Easy check -- get it warm and look for a cold spot on the rad, usually rigth in front of the fan.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!