@Doctor
Listen to me carefully. Your last option should be to repair the head. Its often easy for mechanics and people to blame the head. They just want to make more money. Your problem could be very simple, you'll be surprised. Here's what I would do, since I have just recovered from the same problem with my Shogun/Pajero.
1- Check, recheck, and then check again the cap of the radiator. I changed 4 caps, and the 5th one was the only one that has solved my issue. Do not buy a brand new cap, they will be Chinese or Taiwanese etc, look for a used one from the used parts market, but, it needs to be perfect for your radiator.
2- Cold start your Hilux, keep the hood open, keep the radiator cap open. If within 2-4 minutes, you start noticing bubbles emerging in the radiator water, you have to replace your gasket/reinstall it again. You will get bubbles with the ignition, ignore those. Repeat with a normal temperature engine, not overheated, but when its on normal temperature. See results.
3- Check for minor fractures in all the radiator pipes, they could be microscopic. If ounces leak, your car will overheat under stress. Get all minor fractures repaired. Minor fractures are difficult to detect, as the water will evaporate and you will often not be able to see water traces.
4- Turn your car on, have someone monitor the clutch fan (big fan, not the AC fan). Tell them not to look away. Turn off your car, your fan should stop IMMEDIATELY, there should not be cycles of the fan like when you turn off a ceiling fan in your room. It should be like the fan has applied hard brakes. If your fan revolves and comes to a slow stop, you need to replace your fan immediately. You will blow your engine soon if you don't.
5- Check all your hoses, and see if they are clogged. Do not fill up your reserve water bottle to the brim, it will become air tight, and it will cause cyclical problems in your radiator system.
6- Only if none of these diagnosis works should you look at a head repair.
7- If you can, plan a trip to the mountains, or something steep, or something like in the desert. Do not turn your AC on, if your car heats up, its got nothing to do with your AC, it has everything to do with your engine not being able to take load.
DO NOT LISTEN TO YOUR REGULAR MECHANIC. DO NOT LISTEN TO AMANULLAH, he is good, but is there to make money. Before you go to a mechanic, visit a specialist radiator waala.
Hope this helps. And best of luck.
Rao.