Check out this thread (
https://www.mobileread.com/forums/sho...d.php?t=101734) I had the same problem. I tried firmware updates, restore to factory settings and re-setup the kindle, wifi on/off. No difference. I spoke with Amazon and their solution (as I had gone through all the steps they would have suggested anyway) was to replace the Kindle.
I am not sure what causes it. Our second Kindle does it occasionally but the wife is happy to leave it for now. I did catch it the other day doing a reboot, essentially all that happened was the screen flickered a few times then started up, so the Kindle isn't doing a proper shutdown/reboot, it appears to be a hard reboot. Her Kindle is running 3.0.3 so no doubt a hardware problem.