My Roomba series 500 had developed a hitch in it's git'along. Using the Roomba troubleshooting guide on their web site I determined that the cleaning head module was bad.
From the sound I figured the gears were broken so I just ordered a new module from Roomba: $60. It arrived within a week. Removal of the old unit and installing the new one was a snap. The Roomba is well designed and they make it easy to work on.
After getting the new module I figured I had nothing to loose opening the old one up. Opening the gear case I thought all the little parts would fall out and I'd never be able to get it back together again.
I was wrong.
It came apart easily and was a simple design well executed. Three of the gears were identical and there was no question about what went where.
The problem was caused by the dust that had gotten into the gear box. As the gears mesh they pack the dust in between the teeth. As this packed dust builds up it puts tremendous pressure on the whole system. One of the gear axial in the box wore out its support on one end until it failed.
My advice is to clean this gear box out before the failure occurs. If you're mechanically inclined at all you should have no problems doing it in an hour. Just takes two sizes of Philips screw drivers.