I haven't had much photo/video stuff break so I'm confused where to head with this. The problem is with my Canon EF 28-135mm f/3.5-5.6 IS USM, which has always been a rock for me.

I hadn't taken pics for a while and got out the 10D for some eBay pictures. There was a problem keeping focus, but I figured it was just in my head (I'm kind of a perfectionist with pictures). But sure enough I copied them up and about everything was noticably fuzzy... like 30 auction pictures, all unusable :/

So I tried to do the pics again and had no luck getting a straight picture out of the thing. I wound up taking all the picutres with my Sigma 10-20mm, not my prefered lense for the job but it proved that the camera was working with some lens at least.

When I was shooting with the Canon lens, I'd zoom in fully, focus, and zoom out to frame. The pictures look like an extremely shallow DOF, but the distortion is even across the frame. It's kind of confusing and I finally wound up testing the Sigma and Canon on a resolution chart to verify there was a problem.

Here is some of the stuff I remember trying with the Canon:
extreme fstop settings
remove UV filter
ISO settings
image stabilizer on/off
autofocus on
clean front/back elements (they didn't look bad anyway)
look at sensor/camera (it worked fine with the sigma after all)

And lots of other stuff I can't remember. The last thing I used the lens for was in around freezing weather for about half an hour taking pictures of a snow dinosaur I sculpted, those pics looked fine (all things considered). Is there something else I should try? Thanks for any help and sorry about the huge post, I tried to put everything I did in there.

I'm not a Canon shooter, but I thought the 28-135 is a varifocal lens. Meaning when you zoom, you have to refocus. If this is true then it is your technique giving you your problem.