The overflow hose is going through the throttle body then going to what's supposed to be a return.wich I don't see how it would work because the return hose that leads to the throttle body has pressure also.the way it should go is the return should be tappedd in where the air bleeder screw is aton the radiator .the coolant system can't flow properly if you have pressure fighting pressure it may not be the transmission it could be the throttle body getting heat soaked and causing the issues