Transientmech is on the right path, fit OE , and make sure that components are operating correctly . There is absolutely no reason for V8 3.9 having o/heat problems . I have/had 3.9i in my heavy 110CSW and have used it in 45C (113F) ambient pulling a 5000lb caravan (trailer) in Australia up long steep grades without worry. The only time you might have to watch temps is when operating an auto gearbox model in heavy sand with your aircon on , which a thing in common with most 4wd that i have come across, (not mine as the condensor is on the roof). The Landrover products were very much developed with a close eye on sales to middle east , and with their ancestry , have always operated in extreme conditions. From what you are describing , eg slow travel speed o/heat with a drop as soon as speed picks up , then you need to be looking very closely at your non standard Viscous fan , you should be able to hear the roar of the fan as soon as the temp goes above normal operating temp, this should cycle on and off as you continue with any hard working , as the cooling system should have sufficient reserve capacity to deal with temp rises. HTSH
