I'm afraid my awd/4wd 2003 Montero Sport is starting to run hotter than normal.
This happens when I´m off roading (4wd engage at low speed), traffic in up hill roads, heat can climb really fast and go back to normal just as easy, just need 30 seconds of open road and everything is back to normal.
Is the head gasket burn?

 

I spoke to a mechanic and he told me that a vehicle with a burned head gasket will overheat under any circumstance.
So I'm considering flushing the entire cooling system, put in new coolant and a new thermostat (maybe there is some dirt in the radiator). If nothing changes I will have to replace the head gaskets.

 

It sounds like possibly a water pump. Change the coolant and thermostat like you said and if that doesn't resolve your problem I'd look into the water pump. If you have an oily residue in your coolant and your oil looks milky then its most likely a blown headgasket.