The heater had been working fine until approx 4 weeks ago. Now it has an intermittent problem. It will be working fine and then, suddenly, will blow only cold air into car. This will stop after a day or two and the heater will work for another few days. Today I took the car out on three occasions. Heater ok for first 2 journeys and on the third only cold air. This is really annoying. Any solutions, please ???????????????.