its probably caused by an air leak!...(its a sealed system) check all water connections,clean and reseal etc...reason is when engine gets hot...coolant expands and is diverted to expansion tank and when engine is cool...its returns to the system by suction...but given a choice the engine will suck in air rather than the more denser coolant...this was a major cause of over heating in the mk 4 ford zodiac years ago...remember to check "all" connections...or use a pressure tester... pump up system to 15 psi and watch for leaks or drop in pressure?this will also check for cylinder head/gasket problems ................be lucky