Belt is the only thing I know of. I was fixing my mower today and the same thing happened....after I replaced the two bent blades. Anyways....
Make sure your blades are on nice and tight!
Make sure the belt is completely on its tracks. There shouldn't be too much play in the belt either, from what I understand. If there is, it is easy for it to slip off, and then the belt ends up breaking. Mine broke about 20 minutes after I replaced my blades!
Chuck, July 2006