whats the serial number of your lens? i have experienced the same problem on all lenses with serial number 41XXXXX. i have exchanged the unit several times and found out that all of them have the same issue. Finally got a lens with serial number 39XXXXX and the issue was gone. i even wrote to canon europe hoping they can shed a light regarding this, but they are unaware of the issue they just asked me to bring my lens to their service center.
this is the letter that i sent, kindly use the links as they may exactly describe what you are experiencing and if they do, replace your lens because at this moment canon is still clueless about this.
"I recently aquired EF 24-105mm F/4L, with serial number 41XXXXX. everything is excellent with regards to this lens except this particular issue that i found with the lens:
Handheld shooting with IS switched to "ON", i always get a blurry image with a diagonal pattern whenever i step down (f4.5 and onwards) the aperture. At f4 (wide open) shooting with "IS ON," image appears to be fine, but when you step down to f4.5 (narrow down) and onwards the blur occurs. Less blurriness appears when "IS" is "OFF", which is very odd for this lens to function like this.
However, if you press shutter button halfway then press DOF preview button simultaneously, and then fully press the shutter (without releasing DOF preview) image blur disappears. I believe that the len is at fault here. lens tested on different body (full frame and crop body) same results. I have exchanged the product from the dealer several times but all units exhibits the same issue.
The dealer is clueless and we're on a dead end here, he also doesn't seem to know what to do.
We tried testing the same lens on tripod with "IS OFF" on a (full and crop sensor body) all images at all stops are fine.
Just to inform you i am not the only one experiencing this issue. kindly refer to this links as they are EXACTLY having the same trouble that i have.
For the record it was reported by one user that CANON SERVICE CENTER was able to correct it by changing the FIRMWARE of the LENS. kindly refer to this link.
i hope this issue will soon be addressed. hoping to hear good news from your end. thank you very much."