I did a "big indexing test" today: removed all the books and index files from the K3, which needed a bit of housekeeping anyway after 2 1/2 years
and moved about 650 books in one go from Calibre back to the device. It's now about 10 hours later and everything has indexed properly without problems. As usual, the Kindle was mostly left alsleep and plugged into the wall charger while it was working its socks off (I just checked every hour that the number of books left to index was reducing).
Maybe *something* in some books causing random issues for some people?