OK, I've seriously narrowed down my overheating trouble to when I'm travelling at freeway speeds and going up grades of 10% or more.
Changed: thermostat, flushed and filled coolant, Water Pump Tensioner.
Can a Water Pump not circulate enough coolant (in other words, be bad) without leaking, making noises or otherwise obvious outward appearances?
I promise this will be my last cooling system question! I'm nearly fully competent on Northstar!