Went through two Sigma 120-300/2.8 lenses with heavy back focus issues in 2007, the second had even worse back focus than the first.
Returned it for Nikkor 70-200/2.8VR with 1.7TC and never looked back as I never had focus issues with any Nikon lens I've owned or borrowed.
Even now that cameras can be adjusted for focus issues and the Sigma lenses can even be tuned via the dock it'll take a long time before I touch another Sigma lens, they are making some great lenses now but I simply don't have the time to deal with failing equipment as it frustrates me greatly (one of my many shortcomings).
I hope you have more luck, if it works that 35/1.4 should be a killer lens.