My Roomba had been on the fritz lately. When I powered it on it went forward a few inches and then started backing up in a tight circle. I figured it was a dirty sensor, but I cleaned everything I could see and had no luck.
My coworker Brandon pointed me to the Circle Dance website, which explains how a dirty internal sensor can cause just that problem. I've got an older Roomba, but the wheel assembly seemed the same. The site has great instructions and photos showing how one can fix the problem. They do, however, go through incredible contortions, including removing 10 screws and a hard to replace panel, just to remove a single screw.