>What year lens do you have? HSM or HSM II?? There were issues >with the early 70-200 Sigmas, I believe fro reading pre-2005 >which had the same symptoms. If it is the lens, it should be >reproducible on another camera if you have one.
I purchased the lens a little under 2 years ago. It is a HSM model. I have a D40 in the closet somewhere I could try it on if I can find it.
Here is another tidbit; I was reading on a Pentax forum (I believe) where someone had a similar issue with the same lens but with a Pentax body. They found that the screws on the lens' mount had come loose. I checked the mount screws on my lens and they were indeed a little loose. I carefully tightened the screws and did not have a problem with the lens shooting about 20 or 30 shots with it. I haven't had the time to shoot much but will try to get out this evening and do more.