heater blow problem in Vauxhall Vectra?

I have a 2002 diesel Vauxhall Vectra (new shape). When I start the car the interior heater blower comes on but then slowly decreases in strenght untill it is off. At that point the blower doesnt word. If I stop the car for 10 min and then re-start it, the blower comes back on but only for around 5 mins and then stops again. Any ideas ?