Hello, well if your losing coolant eventually your vehicle will overheat check all the hoses for leaks . If the radiator has been replaced with a new one i doubt it would be leaking there a repair job unless done professionally isnt advisable. Overheating will be caused because there is insufficient coolant circulating around your engine eventually the coolant just gets hotter and hotter and has to escape somehow usually through the expansion tank. We could do with a bit more info really is there any creamy gunk in the oil cap , or steam coming out of the exhaust . You can have a pressure test on the cooling system via the expansion tank. The system is pressurized and left for 10 mins if there is a leak it will be detected by careful inspection.Try a diffrent garage sounds like the one your using is after your money.