Re: help, chain, sprocket
Have you beveled the teeth on the rear sprocket?....That will usually do the trick assuming you have no stiff links in the chain (the chain line is generally pretty close in most installs and you wold be surprised how far off it can be and still not cause a problem)...
Try beveling and I think you will be happy....(Pic below to show you what I mean....see how the sides of the teeth should angle in on both sides?).
Hope this helps you.
Good luck in your build.