Try to return it first as suggested on the last page. If not, send it to Canon. Mine had the sort of the same problem on my 7D and my two rebels. Using at f/2.8 it worked fine outside using Servo, but anything else it was terrible. Indoors it would never focus where it was suppose to. I sent that thing to Canon 4 times in the year that I had owned it and took the very last time for them to fix the issue. Take photos in RAW and send them in with the lens on a CD to show them what is happening. That's finally what I had to do and it worked. It is now one of my favorite lenses to use! If you have the warranty, use it. Call Canon if under warranty so they will at least pay the shipping and you aren't out anything.