My 2001 Galaxy 1.9Tdi is overheating when driving at high speed on the motorway. It is perfectly fine in normal city driving. The temperature never goes above 90degrees in the city but on the motorway the temp starts to creep up when the engine is under constant load. The water level and oil are ok. The radiator fans work. The heaters work fine. The top radiator hose is warm. The bottom one is cold. Could this be the problem? I suspect the thermostat, but a mechanic I took it to is adamant that its the water pump.....and wants