http://digitaljournalist.org/issue0807/tech-tips.html

Chuck Westfall recommends turning IS off when on a tripod for the 24-105L.

I have always turned IS off on my 24-105L because I have seen a noticeable difference when I do not. Not sure that is you problem or if something is broken... just my experience. The only way you are going to know for sure is to send it in to canon service.