I have a problem with the climate control unit on our '04 plate FM12. It has been going on for about a year and my local dealer doesn't seem to be able to rectify it. It will only blow warm air when it is on max temp either on auto or max fan setting. Also seems to start blowing cold when you reach 30 - 40mph. Coincidence? Fairly sure the thermostat has been replaced and also the climate unit itself. Any ideas 