I've just recently had this problem fixed. In my case the a/c control module (big black box that houses the 3 centre swtiches and two dials) was up the duff thus causing the a/c to have a mind of it's own.
I had suspected this straight after the garage that had sold me the car had supposedly fixed a very minor a/c problem <img src="http://www.mbclub.co.uk/iB_html/non-cgi/emoticons/mad.gif" border="0" valign="absmiddle" alt=':angry:'> Well at least they gave me a good deal on the car <img src="http://www.mbclub.co.uk/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt=''>
Trouble is, I've not had the time until recently to be able to give the car up for a few days for the problem to be rectified. Very embarrasing when turning the car on and it sounds like it's going to take off because both aux fans switch on at full pelt. <img src="http://www.mbclub.co.uk/iB_html/non-cgi/emoticons/wow.gif" border="0" valign="absmiddle" alt=':0'>