My guess diagnosis of the problem (and I have been through this) is that since your car runs cold all the time the micro leaks in the system do not show up. Since the car never reaches optimal temperature hence leaks never show up.
However once you turn on the AC the temperature rises and then these micro very small leaks in the cooling sytem start to do their job. A car that only runs on tap water will have atleast 35 micro punctures in the cooling system that only show once the car heats up to proper temperature. Then these leaks suck in air and also drop the water level in the radiator. Eventually this process of more air coming in and more water going out reaches a point where the water in the cooling system becomes steam and then blows up the radiator or hoses. If you don't fix it one day you will burn your gasket. The solution is to tear down every thing.
Firstly pressure test the cooling system with 10 to 20 psi of pressure, check youtube for videos to do DIY at home. You will see under pressure all leaks that will not be visible on a cold system.
If you can't pressure test then my advice is to change all hoses, water bodies, metal pipes and also inspect the water inlet and outlet and tikli (small freeze plug like thing on the bottom) of intake manifold, change the freeze plugs on the engine and change radiator cap. Check if the radiator neck is not bent and making 100% contact with the rubber on radiator cap. Then put a good coolant and you are good to go for another 10 years I guess just change coolant at 40k kms.
It is not much of an expense, all water bodies and all hoses and metal pipes in the cooling system did not cost me much in my car. I think it was 100-300 per rubber hose pipe and 100-300 for each water body.
Also diagnose gasket problems, are their any small hundreds of bubbles appearing on the radiator neck when the car is started and radiator cap is off after 5 minutes of running? Are there any Vaseline like gooey stuff on the engine oil cap? Water coming out with pressure out of the radiator neck happens only when the gasket is like in 'really bad' condition. I suppose you won't have that problem.
If all of the above doesn't fix the problem then I suppose their might be a hair line crack in your head, get it checked and I know there are people out their in Karachi that have the facility to check this in Karachi (although I don't know any). It is an advance process requiring special tools (check youtube) and your mechanic might be able to tell where do they send their heads for checking for hair line cracks.
Edit: Also check if the radiator fan blades are not warped and the shroud is tightly snug with the radiator. Warped fins will not pass air through the radiator and if shroud is not tightly snug it will instead of pulling air "through" the radiator will pull from the sides and not cool the coolant.