100-400 II IS and AF Issues

Hi All...I recently purchased the new Canon 100-400. At first I was completely amazed by the lens. After having the MKI for the last few years, I was stunned by the incredible IS and the response of the AF. However, after using it for a few weeks, I've begun noticing a couple potential problems. I'm wondering if anyone else has seen similar issues. The biggest problem I'm having right now is with AF response both in Single-Point and Servo modes. The AF seems to just give up, even on fairly high-contrast subjects in relatively good light. Shooting smaller birds at distance, something that I had no issue with on my MKI, is almost impossible as the lens just searches and searches, eventually giving up or giving me an out of focus shot. When I do manage to get it to lock on, it seems to jump between the subject and the background over and over again, such that I end up missing 50% of the shots, despite the focus point never leaving the subject.

The second issue is with the IS. At least two to three times each time I go out shooting, the lens will take between 1-3 seconds to engage IS after I press the shutter button. Sometimes it wont engage until I release and re-press the shutter. This doesn't seem like correct behavior. Has anyone else noticed either of these issues, or did I get a dud? I'm planning on contacting Canon tomorrow, but I figured I'd ask around a bit first. Otherwise I'm still stunned by how sharp my shots are when it DOES focus correctly. Even with the 1.4x extender it's at least as sharp as my MK1 was bare. The 2X is also usable (if you're willing to live view focus).

Almost sounds like it's a connection problem between the camera body and lens? Have a backup camera body to try?

Mine usually does fine with the 1.4xiii + 5dmkiii, however, in lower light, I still want that 500 F4L II, as sometimes it will hunt say shooting a hawk over brown grass/field at long distances...

It certainly does not sound right, does this happen often enough you can take it to the store and try it out on another body?

It's probably the quickest way to isolate where the problem may be.

I've tried it on my old 60D and I noticed similar AF issues. The IS issue seems very inconsistent when it occurs, but I haven't had that one happen on the 60D yet. I'm gonna give it a try on a 5DII as well, I think just to see what I notice.

Could be the lens interface contacts, as you already tried on a second camera, and it was working fine when you got it.

research how to clean the contacts properly, or better yet, call Canon and ask them what to do. If indeed they mention cleaning the contacts, ask them to send written instructions by email too.
More likely they will want to inspect and repair the lens as it's under warranty, and will send you a shipping label

....and that is if indeed you still having problems after testing it on the 5D2

Could be the lens interface contacts, as you already tried on a second camera, and it was working fine when you got it.

research how to clean the contacts properly, or better yet, call Canon and ask them what to do. If indeed they mention cleaning the contacts, ask them to send written instructions by email too.
More likely they will want to inspect and repair the lens as it's under warranty, and will send you a shipping label

....and that is if indeed you still having problems after testing it on the 5D2

I noticed a similar problem with my lens that I just received and took to a workshop. It would just freeze up intermittently. It was suggested to me to try cleaning the contacts and it seems to have solved most of the issues, but I need to take it out and shoot a lot to see if it recurs. I used it some this weekend and only noticed a "maybe" issue once. This was on a 5D MKIII. Now that I've seen it mentioned here I feel like it may be "real" and not ineptness on my part. Otherwise I love the lens.