this all sound like what happened to mine exsactly the same symptoms and i wasted so much money doing what your about to do changed the water pump as i though the impelers could be broken, wasnt that but changed ut any way while it was out then changed the thermostat and still the same also changed the blue temp sensor then changed the expansion bottle cap still he same. then finally done the head gasket then all was good. after doing the head gasket the water started coming out the overflow agian but very slightly so i went to vw got another genuine cap and new bottle then its been perfect. s