I have also heard about this float that is in the thermostat, worth checking as they get lost pretty frequently.

As for "fans on a switch" just dont bother, let the temp switch do it itself, if you want it on earlier get a lower starting one, but neither of these things will fix the underlying problem. If your car is getting hot with the fans on, you can have them come on at 70C/60C/50C, its not going to fix it!