Yes that can be very annoying! I was recently trying to convert a DRMd book from ereader format to epub and no matter what I tried it would crash the reader. I ended do a conversion to mobi which seemed to work on my kindle2 and then converted the mobi to epub which finally worked. This whole sequence was a total PITA so my conclusion is to never buy ereader books again.
P.S. the lockup/restart is not limited to Sony either. I had it happen on my Kindle2 and it screwed up my recently read order (cause it lost the current time and re-ordered everything) and I've had my Astak Pocket Pro lock up several times, but it doesn't seem to loose the information when it does.