Nikon D50 with Sigma 70-200 f2.8 APO DG EX Macro HSM.

I have, since I bought the Sigma lens problems with the focus/sharpness of my pictures. All from f2.8 to f6.3/f7.1 are not in focus/sharp! With any light or in every mode of the camera.

Camera has been checked (kitlens 18-55 works fine), lens is (almost) new.

Has anyone any idea what the problem is and what I can do about it before bringing it back to a Sigma dealership?