I had the 135mm and had a simular problem with a newer body and a 1st generation lens, seems they had conflicting software problems, what your describing sounds like the problem I had, the software in the lens was not able to resolve the focal point as well as the AF sensor in the camera, I sent it to a repair shop and they were able to upgrade the software in the lens, and I have never had a problem since.

As far as the quality of the lens, it has been a very good performer and I have used it at numerous weddings over the years.

Dave