I think that you're right on the money, Mpolli. AC adds heat to the radiator, and it is important that both fan systems are working as designed. Most likely, one of these is at fault.

If those are good, the problem may be that the radiator passages are calcified due to using hard water in the system. Thermostat would seem unlikely if the engine temperature is well-regulated without the AC running -- but we don't know for sure that it is.