This is exactly what was wrong with mine! by: Andy
Hi, I wrote an E-mail describing my issue--it seems the drive gear was floating in the bushing. My machine hasn't been able to climb in a year, and this is probably the culprit.
Curt not only answered my question promptly, but he pointed me to this page. This page describes exactly what is happening with my Aquabot, and the parts are now on order! Here's to looking to a clean pool again.