Canon 100-400 Problem

Hi all.
I use a Canon EOS10D and 20D with a 100-400 L series lens bought new one year ago.
Recently when used at 100mm focal length something goes wrong inside the lens: photos made at that focal length appear to be made with 400mm focal length. The result is often a grossly overexposed shot, since the camera "thinks" to have a 400mm instead of a 100mm and consequently uses a lower F number (like 5,6 instead of 7).
The curious aspect is that moving a little the lens to, for example, 105mm resolves the problem (for now at least). Is there something I can do to solve the problem?
Thank You for Your help.

Unfortunately that sounds like a send to Canon for Repair problem. I had similiar (although not the same) problem with my 28-135 IS awhile back and that was the only solution.

I have never experienced that problem with my 100-400.

Since it is probably the electronics, I suggest playing with it for about a week in case the problem just goes away. If not, call Canon service.

Just my opinion,