Upload & Sell: On
I don't recall having seen any posts about the d7k having issues with Sigma lenses. But, there are a lot of posts about AF problems, without identifying the lenses. I tend to believe that there are some bodies out there with issues.
My d7k doesn't work well with my Sigma 100-300 f/4 or 120-300, but it doesn't involve the jumping and jerking that you describe. The lenses just won't focus properly at any distance. I haven't bothered to test my other Sigma lenses or do anything with the d7k. It seems to work fine with a Nikon lens of 200mm or higher. I haven't sent the lenses in to Sigma because they work fine on all of my other bodies.
I know that isn't real helpful, but it's the best I can do for now.