HASSAN IQBAL

Hi, just wanting to know if anyone can point me in the right direction. I have a 2006 Subaru Impreza wrx 2.5 turbo. I’m losing water from the Mishimoto and starts to overheat when The car is driven a bit hard, but if you drive the car normal it will drive forever no issues, but a mechanic has tested and said it’s not the head gasket. Thermostat and water pump have been checked and are all fine. Any help would be appreciated thanks.