I have had my 650 for about three days now. I have been having the same problem. Mine freezes randomly, sometimes while turning pages, sometimes while accessing menus, etc. After searching this forum, I found that there's a thread about this for the 600 - I tried some of the things in that thread and they have helped.
I took out my SD card and loaded a few hundred books to the internal memory. No freezes or glitches at all, even while doing some fast page turns. The device worked easier and faster overall.
Right now I'm about to try out my SD card again, this time loaded with epubs only (last was a mix of epub and PDF) in case some of the PDFs are really large and hanging something up. Will report back if anything changes....
The thread for the 600 -
https://www.mobileread.com/forums/showthread.php?t=56621