I had the same thing happen on my girlfriends Sigma 18-250mm OS lens on a 450D. Also tried cleaning the contacts but that didn't do much and I wondered how they would be dirty anyway since the lens is always on the camera. It turned out to be a broken contact cable that bends when you zoom. Similar to this: http://doekle.nl/canon_17_85_IS_repair.html Your case sounds a lot like that. This site also has info and links to similar cases: http://martybugs.net/blog/blog.cgi/gear/lenses/Canon-17-85mm-Lens-Issues.html

Hope those help. The repair cost me around 130€ including parts since the lens was out of warranty. A bit steep IMO but the checking and estimate was 80€ so the added 50€ wasn't that bad. If I had known the cause earlier I might have tried to repair the lens myself if I had found cheap parts in eBay.

EDIT: In your case the lens is much more valuable though so might be better to have it fixed than poke around yourself.