Sigma 18-250mm, AF issues?

[gl600]

Hey,
i just received my Sigma 18-250 from Amazon. I was very excited and immediately started playing around. Then I noticed that when i was zoomed out to 135-250mm the AF would focus 99% of the way and then the last little bit it would start clicking really softly as though the gears were malfunctioning. If I nudge the focus ring a little bit, the AF will kick in and focus properly... but if I don't touch it, it will continue clicking and will not complete focusing.
Has anyone heard of this issue?
I will probably just send it back in anyways, but I figured I'd throw this out there and see what happens.
Thanks.

P.S. Why does my Nikkor 55-200 capture a closer/larger image @200mm than my sigma 18-250 @250mm???

 

[PhotoXopher]

Mine did that for the first time yesterday morning, I think it was due to a very difficult to focus scene because I've never had a problem otherwise (or since then).