How can I fix me Citroen C5 climate control?

My climate control has become very random. When I start up, it works fine for a while, but when I select the demist front windscreen, the fan poweres up as normal for about 5 seconds, then dies to nothing, and the fan picture on the control corresponds. then randomly within next 5 mins, it comes back on again?

It is not limitd to selecting the demist button, but that is where it happens most often.

Any help is greatly appreciated