I have a similar problem on the 50-500 where it won't focus between 200-300mm at certain times. The lens will focus at close distance, but not far targets. The lens has been back to Sigma 3 times, and they still haven't fixed the problem. They say it's to do with the auto focus motor and have re-calibrated everything on 3 occasions, but the problem soon comes back so I just live with it. Your problem sounds similar. In my case, it seems to only be a problem when the lens is used on cold days, it's fine at the moment with it being the summer so work that one out! If you have a problem, and especially if the camera is under warranty, I would send it back to Sigma to see what they say. They charged me £75 the first time I sent the lens back but all work after was done under warranty as the first time didn't fix the problem.
Good luck, I hope you have more luck that I have!
Equipment: 2x Canon EOS 50D; Sigma 10-20 EX DC HSM, 50-500 EX APO DG, Canon 24-105 f/4 L, Speedlite 430EX