That is a serious problem that must be fixed asap. From what you are saying, the viscous coupling has failed. I thought the viscous would lock up if it fails, but I see I was clearly wrong about that. the hotter the engine water temp is the tighter the coupling gets to allow the radiator fan to spin efficiently to draw air through the radiator. At high speed ( I'm guessing around 80Km/h ) there should be enough air forced through the radiator, but in traffic you will cook that motor.
Unfortunately the only place I know of, other than Nissan themselves, is a place in the Cape ( near Belville ). I'll have to hunt down the places name, but Elaine was the person I bought mine from when I was there last December.
The problem is that there are about 3 different viscous and fan combinations. I would suggest taking out the viscous and VERY accurately measure the PCD's of the 6 nuts that hold the fan blade on and the other coupling with the 4 bolts. You might as well measure almost everything you can and send the drawing to her to match up a replacement.
Their prices are reasonable and I'm sure they can assist you in getting it up to Hoedspruit.
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Suzuki TL 1000R "V twin" (Growler) - SOLD