Could it be that indexing is the problem? Every new Kindle-owner wants to load his/her book on right away.
I loaded appr. 1800 books on my new Kindle in batches of 2/300.

I soon found out, that indexing took a lot of spirit and batterylife out of the Kindle. So I let it stay connected to the PC while doing that and gave it a rest.
Sure, I took the updates as soon as they got out and with every update my Kindle ran smoother.
But in retrospect; I wonder if those crashes and reboots I had in the beginning weren't caused by my loading all these books and the Kindle indexing that load?