kindle 3 freeze and reboot randomly, why?

Hi,

my old but loved kindle 3 (wifi version) has started to freeze and after some seconds reboot randomly, and often after this it keep to be freezed (a sort of boot-loop google nexus style). I tried:

• to do a factory reset: no results;
• to change the battery: no results.

Can you help me? I read on internet there is a way to reach the recovery mode and then doing a hard reset, but my it seems my kindle doesn’t want to do it.