just bought a cossie and heaters were very cold so i bought a new thermostat when i took the housing off i noticed that there was no thermo in it put one in and i got heaters very hot when standing still but im losing heat when car is driving the water is comming back into the header tank no problem so i dont think there is a blockage anyone got any help .......the guy i bought it off said he just had the head gasket done with a new timming belt i can see there has been a new timming belt and new pulleys but cant see if theres a new head gasket done please help