@PoP: Just to make sure you're both talking about the same thing:
* Open a book (A)
* Open a different book (B)
At this point, it's sleeping with cover A instead of cover B
And now it properly shows cover B!
Provided you leave enough time to process the cover if those are both new books, is that what happens?
In which case, some more questions:
* How many books are there on your device?
* How's the surface temperature of the Kindle? Hot? Cold? In the usual range?