[syriss]

This may sound to simple, but i had the same problem and it turned out to be air in the cooling system. Because of the relationship of the radiator to the engine, if the coolant reservoir runs dry, air will get into the system. The only way to get the air out is to bleed the system using a bleeder valve located on the thermostat casing. It can take a while to get all the air out but it did fix my problem and i hope yours is as simple.