I have a 02 gtp with 80000. I am losing coolant but without any noticable leaks. About every 3 months the light comes on(yes I have replaced the sensor) and I add maybe a 1/3 of a gallon and lights out. I have had the system pressure tested cold and hot. Holds good pressure either way. No milky build up on dip stick no smells. Has me befumbled. anyone else have any suggestions?