I am so stumped on this one HELP

Ok my problem is I am losing radiator fluid which looks to be at the bottem of the rad on the right front of the car, this is the second new rad, I am told it might be a head gasket blowing compression into the cooling system but the oil is fine and so is the coolant fluid, the temp gauge is screwy but I installed a mechanical one to the bleeder valve before the thermostate and everything is cooling properly, even two garages tested the comp and its cooing fine, I also replaced the temp sensor and sending unit, so is it the rad leaking, its only the resovoir that goes low and is it possible the head gasket even though the car is fine otherwise, my question is what is it and is there a definite way of finding out if it is the head gasket. thanks