I have an ongoing interior heating problem which no dealer has managed to fix. With Auto selected and say a temp of 22deg selected, heater comes into action once engine has warmed - all fine. After a varied period heating starts blowing cold. Increased temp demand usually has no effect until either a much higher temp is selected eg 27 deg or more often HI has to be selected and then only control is the fan speed. Similar situation if manual is selected from the outset. Anyone any ideas? Perhaps a faulty thermostat/sensor somewhere? Very annoying.