dexter, I think your problem is slightly different from chetz.
Your aircon blow hot air and temperature slowly creep to the 4/4 mark (red zone).
I have to assume that the low coolant alarm did not come on. If it did you would have mentioned it.
Normally, if there is low coolant, the temperature will rise without warning.
btw, have you change most of the cooling system parts like thermostat, water pump, hoses and expansion tank?
If you have not done it, better do it soon as your car is about 10 years old. Most of the plastic will becomes brittle.
Coming back to your problem, I could only say the possible cause is:-
1. airlock in the system. - please check for air bubble and bleed the system properly. there is a bleeder cap for bleeding near the radiator.
see picture below.
2. If the aircon is blowing hot air, it could mean that the electric aux fan is not working properly. check whether it spin fast when the engine is hot. Insufficient cold air blowing thru the radiator will cause the engine temperature to rise.
3. check the clutch fan to see whether it is spiining fast when you rev the engine. You can try to use a newspaper and roll it up. try to stop the clutch fan with it.
