nokia lumia 520 freezes what is the cause

we have brought a secondhand nokia lumia 520 works well until you put it down, when you come to use it again it has frozen and will open to windows page or even turn off completely unless we take the battery out for a few minutes and put it back in again. Can you tell me the possible causes.

I experience this more on windows phone 8.1 developer preview

Unclear, potentially some bug somewhere.

Have you tried a hard reset? (erases all data on the phone)