Your problem is probably the switch unit( sending unit ) that turns the fan on. The reason the fans come on with the A/C is that it is wired to do so to compensate for the loading to the engine with the A/C on. You want to replace your thermostat ANYTIME your engine overheats to the point of throwing out steam. Repeated overheating of a engine can destroy it. If you have to drive it, only do so with the A/C on to ensure cooling.