Why will my Montero start to over heat while driving at low speeds?

It could be the that the water pump impeller has started to wear, therefore the flow is greatly reduced. Increasing the rpm jumps up the flow, thereby cooling off the engine. I would also check out the cooling fan, as the fan clutch may be starting to slip and not providing sufficient airflow at low rpm.

Thank you for your replies. At first I thought it might be a bad gauge. The morning after my posted question the temp. Stayed in the green on a 200 mile trip down to the southern part of the oregon coast. I drove around there some as well. Two days later I had to make the trip back up and return immediately. Keep in mind the whole time I was pulling a 10ft trailer for work that I was doing down there. On the way back up everything was still fine. But, when I headed back down for the second time. About half way there I made a few frequent stops for some lunch and a mini market, then stopped at a card lock fuel station to fill up and put a little oil in. Once I got back on the highway again it started to heat up.I managed to bring the temperature down by easing off the gas but maintaining the speed limit of 65. It stayed in the norm until I started to head up the coastal mountain range. It would heat up as soon as I climbed and cool when I went down. It did this the rest of the trip. It seems to over heat like this once I work the motor harder than normal. But this isnt always the case. If I dont drop the gears down and let it struggle in a higher gear it seems to run cooler. Once I arrived I turned off the motor. Then turned the key to the on position with out starting it. So I could see what the gauge read. It shoot back up to what it had been reading. Which was above average temp but not in the red.I noticed it was cooling down very quickly. Like almost immediatly, which I thought was weird and unlike any other experience I have had with a car over heating. Then checked the fan. I noticed it had a pvc plastic like clutch fan off the crank that I was able to spin by hand without much resistance. A small electric one in front of the transmission radiator and motor radiator was off.I thought that should of been on? You tell me. The next few days I putted around back and forth from my customers home and hotel, staying in the normal temp. The entire time. Once I headed north again for home the temp. Never jumped out of the normal range. Even when I decided to really get on it climbing the hills.
Other simptoms- every time a/c is on it over heats and would xtremely over heat if I let it. Then cools when turned off, dropping gears to speed up even at high speeds, cools when I come to a stop, now once it gets hot, the temp rises when I try to gradually give it more gas to speed up, temp. Gauge jumps sporadically but never into the red, fluids seem to be normal other than the over flow is a dark color but not in radiator andnot the same as oil being mixed (have seen what that looks like)it seems to be happening more often but not every time. Need advice before something happens.
One more thing I didnt mention before. The first time it over heated I was stopped for a while with a/c on and when I got into the car the temp was in the red. I turned the car off in a panic and turned key to the on position and the temp dropped very quickly.

KHlow is correct in his/her analysis cooling fans need to kick on when the AC is running, however that would more impact the temp and efficiency of the A/C.

The fan free spinning when the truck is stopped is a concern, however. If the Montero is hot and the ignition is off, that fan should have significant resistance on it, as the fan clutch will be fully engaged. I think this is your likely culprit.

I have a 02 Limited and had similiar overheating issues. Replaced the radiator, all hoses, thermostat and bit the bullet for a water pump. Did everything myself except for the pump. It runs cool as a fan now.