I 2nd the case question, as it is a known issue. See discussion here: This cover causes the Kindle to freeze up, and or reboot!
A customer qutoed a representitive as saying
"There have been some forum discussions regarding the non-lighted Kindle cover, and our engineering team is looking into this. Regardless, if anyone is having any problem with an Amazon-manufactured Kindle cover, please call us at 877-453-4512 or 206-922-0844. We will be pleased to replace it for free with a different cover or accept a return for a full refund, no matter when the cover was purchased."
If you are using that cover, I would suggest, that you test it out (removing the Kindle from the cover is said to clear up the issue right away) or call to see if that offer is legit.