I had this same problem on the 2002 900 that I just bought....when I got to looking at it I found that the little locking humps that would hold it in position at the joint were all worn down.....so I swapped the mirrors off of my 2001 750 Sport.....which I am about to sell...lol

have you checked this area to see if that's the problem? did you buy the bike new? if so take it back and get them to warranty it?