Lens ring keeps going back and forth, keeps focusing in and out. What can cause this? T2i 17-50 2.8

[TreyfromECU]

Recently I have been having issues with the auto-focus on my camera.  The focus ring just keeps turning back and forth and takes forever to focus, if it ever does.  It gets even worse when it's dark.  Any answers to what might be causing this? maybe sensor issues?  The lens seems clear and I cleaned the filter.  I a using a Rebel T2i and a Sigma EX OS17-50mm f/2.8.  Any suggestions would be helpful, thanks!

[hsbn]

You can try to clean the lens contact. Have you dropped your lens or something? You may need to send it in for service (Sigma has 5 years warranty).