Thanks for the comments and suggestions.
I have defragged and scanned the memory and SD. No faults reported in that process, but in a few days I will advise if doing this has helped the problem.
It happens on different books, all of which are mobi/prc format, so doesn't seem to be one bad file. I noted the comment on fonts sometimes causing this problem, and so if it persists I will try changing that next.
I also noted the SD card file system is FAT. I can't see any reason why that would make it more freeze prone than FAT32, but it's another thing to try changing I suppose.