Well, I think definitely a lens/camera communication issue. I had this with a Sigma 28-300 which was part of a used kit, and I promptly sold the lens.

I know Sigma makes some nice lenses, but over the years I have had two Sigmas of which both gave me mechanical failures. Since then, I just avoid Sigmas totally. I have also found in some cases that although the Sigmas are often just as sharp as the Nikkor counterparts, they tend to suffer more from other lens flaws, like CA and distortion. They have quality control issues. Some people say as many as 1 in 3 Sigmas are misaligned or have a similar problem. The inconsistency is what irks me. That means you have to be very careful buying them used, and as a result means they have much lower used value. The Sigmas' aperture ring runs on a little steel ball that clicks into grooves on the inside of the ring. Once that mechanism gets damaged, it is a nightmare to get permanently repaired, as it simply finds a way to slip out again every so often. The rubber and paint on older Sigmas turns into glue at some point. Most Nikkors will last longer than a lifetime if you look after them carefully. A good Sigma might last 15 or 20 years, if you are careful and treat it well. Anything longer than that, and you are in a grace period.