120scruiser replied:

It isn't the fan clutch as it usually only works up to about 60km/h.
The faster you go the hotter it gets usually means coolant flow. Maybe the thermostat is stuck open. I would start there.
If it gets hotter the slower you go, such as idling around town, that would mean air flow like the fan clutch.
Definitely coolant flow.
Either thermostat stuck open or water pump impellar corroded away. (highly unlikely but I have seen it before).
I would get the new radiator serviced. During a service they take the tanks off and put cleaning rods through each tube to clear any blockages. While the radiator man is doing that remove thermostat housing and flush with the hose continuously into the block. Also take a heater hose off at the tap and flush both ways through there with heater controls on hot. Put in a new thermostat and serviced radiator and you will be apples. Don't forget coolant and test for stray current with a volt meter.
I would be very surprised if your radiator isn't blocked or partially blocked.

Cheers
120s

Horacehighroller replied:

G-day Peter,

When the engine is cold (first thing in the morning) check the fan and ascertain how hard it is to turn by hand (with the engine NOT running of course) - It should be firm ish.

After starting & running/driving for 5 minutes (so coolant definitely not over HOT) stop engine and again turn by hand - Should be loose such that you can actually spin it.)

When engine is working hard and VERY hot (such as described in your post) again perform the same test. Should be as hard or harder to turn than original test.
If it spins easily the hub can be re-charged with product available from Toyota.

Good Luck, Peter L

Member - DOZER replied:

Pete
the fan hub looses its viscous oil for some reason....it is a real common problem... that only shows itself when you work the motor enough to get it hot (towing up hill or pushing it along at 110)
Another way to check the clutch fan is to put cardboard in front of the rad and idle up...watch the temp climb...at about half, the fan should cut in with a whirring noise...if it doesnt, a trip to Toyota with some money will get you two tubes of silicon oil. Use both in it.
Andrew