You mean text memos, right? Not annotation notes. In any case, the following should work.
Writing text memos was causing my 650 to reboot. A hard reset fixed it. It was caused by a 'bad' book on my ereader. The book caused general instability, and not only when reading the particular book.
So clean all the books off, do a soft reset (reformat memory from menu) then if that doesn't work do a hard-reset (the process is described on this forum).
I think that will fix it.
On a side note, the latest version of Prs+ allows you save annotations and text memos on your PC. Also you can flip between text memos and books at the flick of a switch with some creative key-binding.