It sounds like you might simply be hitting the limits of what is possible with the files if you get banding so easily in them - esp the case if they've already been editing and prepared (and thus lots of data already lost) and then further resized in some manner (again more data loss).
If you can't post an example its hard to know if its a limit from the photo or just an order/method of editing that you are applying too strongly.
I like to denoise in Lightroom3 (has a useful denoise function) when finished with whatever else in there, before sending it to CS5 to resize, and then use smart sharpen last thing.