No problem with the Kindle freezing. My Sony PRS-300 does freeze on occasion. I think it is due to the slow processor not being able to handle large books. I know that calibre allows you to set ePub conversion to keep individual html files within a large ePub to 260 KB or less. I've heard that books which have html files larger than 260 KB can cause problems on Sony readers.

BTW, an ePub is basicaly a compressed/zipped folder containing XML and HTML files and various info for a book. Sony readers seem to be very picky about the structures of those files and not very forgiving if the book was produced with non-standard ePub info. Calibre conversion seems to clear up many of these problems if/when you encounter them.