the sensor in the rad that turns on the fan may be shot. you can try running the fan all the time (when the car is running that is) by connecting the two wires that go into the sensor in the rad and see if that fixes the problem.
if the problem is still around when the fan is running all the time then check the thermostat (by check i mean replace, they cost about $5), if it's still a problem think about the water pump.