I do use the Sony reader software to load books on to my computer. I've never run into this problem before, and have read some pretty large books. I very rarely use the larger font sizes though.
For grins, I tried to access my copy of the bible again. It let me access the table of contents and turn pages in medium size this time. But when I tried switching it to large size, it said "formatting" at the bottom and I got the spinning arrows. After about 3-4 minutes my reader spontaneously re-booted.
As I said, I did use the Sony software to load the book, but it appears to have lost the formatting for this book. When I get a chance, I'll re-load.