I think these cases have some hard pressure at some point or causes the Kindle case to get some bending this causing the contact problems in the motherboard inside. Kindle 3 casing (device case, not cover) are too fragile.
I use Tuff-Luv case with stand (
http://www.amazon.co.uk/Tuff-Luv-Lea...9914202&sr=8-4) and never experienced any rebooting issues.