Well, I'm very relieved the general consensus is that it's an AF-C issue and not a physical lens problem. I'd just purchased the lens (used) from B&H and wanted to rule that out before "window of return" ran out.
funny how many ideas there are!
WHat I see in the exif is that you are shooting wide open at f1.8 and 400/640 shutter speed.
Maybe stop it down a little bit? I have not shot this particular lens, but a lot are sharper stopped down.
