I know that I am resurrecting an old thread but I have a theory.
Where the thermostat sits at the front of the head, there is a small housing that bolts to the front of the cylinder head that the thermostat sits in.
Looking down into this housing with the thermostat out, it appears to have a "seat" that the thermostat closes off when fully open to prevent the coolant from bypassing the radiator.
The "seat" area in my car is fairly corroded, and I have the same issue on long hills.
I believe that the coolant is designed to circulate around the engine block and head only until it heats up to operating temperature, when the thermostat opens and the disc on the bottom of the thermostat "should" close off this gallery.
If corroded like mine, this can no longer seat and allows some of the water to bypass the radiator and just circulate around the block and head, creating an overheat condition.
The good news is that this part is replaceable, I dont know how much or how available it is but since Nissan have sold millions of TD27's around the world, it should be available.
Has anyone explored this before?
if I have the wrong end of the stick, please let me know.
Tony