It is a lot more hard to nail down a solution when it is an intermittent problem experienced by a subset of users. Kobo asked those of us in the beta for this information (if possible).
- What book were you reading?
- What file type?
- How many books are on the device? Main memory? SD card?
- Where was the book you were reading located?
My freeze happened a while back, so I didn't remember what book, etc.