Turns out it was a broken thermostat, $7 to fix. I guess the key things to notice were the overheating at speed and a cold radiator and the suddeness of the change. The hot upper hose threw me a little. The thermostat must have been open enough to allow some hot fluid to pass but not enough for the radiator to cool and then recirculate. The brass bridge on the thermostat was broken in two.

Thanks all,

glenmore
1991 300CE