I don't use covers.
As I mentioned in my first post, I've contacted Amazon already and we went through a factory reset.
I did have the latest patch at the time.
What I did after resetting everything was to copy books back onto the Kindle a few at a time and test the highlighting as I went.
Eventually I found a point at which crashes started creeping in again. It happened to be a time when I copied in books with previous highlighting, so I removed the highlighting from those books and then experienced less crashes for a bit, but the crashes started creeping in again.
I did ask Amazon support if this could have something to do with indexing but they didn't seem to want to comment one way or the other on that. My first suspicion was that there was somehow a link. How can I establish if indexing is ongoing for a particular book. A random search has been mentioned, but what specifically should I be looking for? I wouldn't mind establishing if there is a connection between indexing and these crashes.
While I have only a few books on the Kindle I can do as much highlighting as I want on multiple books. The problem only seems to occur when I have quite a large number of books on the device.