I'd guess, from when this lens was introduced, that it is out of warranty? My web search did not turn up any reference to what you have described. Whether or not in warranty, I'd suggest starting by contacting sigmaphoto.com to see if they have any suggestions. FWIW, I have the (newer version) 50-500 OS USM on a 7D and am very impressed with it (first non-Canon lens I've bought).
Answers & Comments
I'd guess, from when this lens was introduced, that it is out of warranty? My web search did not turn up any reference to what you have described. Whether or not in warranty, I'd suggest starting by contacting sigmaphoto.com to see if they have any suggestions. FWIW, I have the (newer version) 50-500 OS USM on a 7D and am very impressed with it (first non-Canon lens I've bought).