First thing -- you didn't note the year of your car. Is it an '86 or later? These have a notorious Compensation Board behind the temperature gauge that fails and makes the gauge become erratic and unreliable [pre-'86 cars don't have this C.B. and their gauges are generally reliable as long as the voltage stabilizer/regulator is working; or the temperature sensor is good].

Anyway, how sure are you that your car is overheating, as opposed to just warmed up. This should be the first order of business. I recommend one of the hand-held, Infra-red (i.e., remote sensing) thermometers like the RayTeck kind sold by IPD, Sporty's, Griot's Garage, etc. Use this thermometer at the housing of the thermostat (to get a determination of the engine's real coolant temperature), and the intake and output tanks of the radiator (to see if it's really working, and to get an idea of whether there's sufficient flow through the radiator).