Hi all. I have a 2013 Passat Alltrack. The heater is playing up. When i set off, all is OK. About 10 minutes into a journey the heater stops blowing through the vents. The fan is still working. If I stop and push random heater control buttons it all works again. 10 minutes into the journey, all stops again. It's as if the vents are closing. I changed the pollen filter and it worked for about an hour, then all stop again. Any help appreciated. Thanks.