Have had mine rather longer than you. Have taken thousands of pics with it and never had any of the difficulty that you describe, so it would certainly be worth getting yours looked at.
Here is a hand-held shot taken on a local building site earlier this week using the 5D with a 24-105 f/4 zoom lens
They are just record snap shots for a local web site, no special effort to keep them sharp, but they look OK to me and I have seen many as good or better from other 5D users
-------------------- Chris
My memory is getting worse . . . and my conscience clearer