I haven't encountered that issue yet, but last night I noticed my K4 slowing down quite a bit. No new books had been added in a few days, so it shouldn't have been indexing. Then it inexplicably froze while reading a book. I had to hold the power button in for half a minute until it restarted and then everything was fine. That is the only time I've had any issue at all and it seems to have quickly corrected itself.