A couple of things. Recrisp, I like Peter always have my lens set for full focus range. Since the two ranges are under 15m and over 15m, I just let it use the whole range. But I like Peter am confused about the problem you are experiencing since one I catch focus on an object, it seems to keep focus on that. I use AI Servo and once it locks, it stays locked unless I come off the BBF button and find another object for it to track. While it is not fastest focusing lens, to me the 85 1.8 is that lens for me, this lens still seems to focus reasonably quickly. Given I don't have the 400 you have and it is supposed to be a great BIF lens, this one has not let me down.
Now Wagaboo, I too was worried when I heard people say it is a great lens for the money, I spent a great deal of time doing research and found that in a majority of cases, this lens is reviewed by people who use the lenses as as sharp, or almost as sharp as lenses made by Canon and Nikon. So not regarding the money, there are very few lenses that even reach this far, and when they do, this lens is on par with those lenses. And they cost CONSIDERABLY more.
So get the lens, you will be very happy with it I am certain.