check the bottom rad hose it getting hot when car is a normal operating temperature. also check radiator is heating up as it should be, top first. a thermostat is cheap part could try it and see what happens. the fan should come on when temp gauge shows a little over half way, fan should come on and bring it down. the white gunk is a little concerning but keep an eye on it, could be a little condensation or head gasket gone. because engine has overheated so many times it can cause head gasket failure