Climate Control Fault

The climate control in my car has an annoying habit of periodically blowing out hot air for no good reason. Even with the climate control OFF (not just in economy mode but off) it sometimes switches to hot air coming in through the vents rather than ambient air. It does this for a few seconds, and then it's back to normal for a few minutes, then it does the hot air thing again...

I've lived with this for about a year as most of the time it doesn't bother me, but I'll be on holiday with some friends in August and I'd rather not have to keep apologising for my car if the air-con periodically gives them a few seconds blast of hot air!

Any ideas as to what the fault is and potential cost to get it fixed? Is this something I need a main dealer for or should an independent be able to sort this?

Thanks for any advice :-)

Edited 14 July, 201312 yr by Ultrasonic