I set off home from work tonight and the heater wasn't working. Shortly afterwards the temp gauge went through the roof. Pulled over and let it cool for a while. Radiator was cool, the pipe entering the radiator at the top was very warm. I set off again and the same thing happened, but I got a blast of hot air from the heater at high revs. Kept it in second gear and high revs for a while and the heater worked fine, and the temp gauge came back down to normal. Is this a knackered water pump or a knackered something else that isn't sending enough electricity to the water pump at low revs? Thanks in advance for any help. It is a 2005 WRX blobeye, 98,000 miles.