How can I prevent from freezings?

[frogi16]

Hi,
I recently bought a HP Prime and I'm currently discovering its options. In 30 minutes my calculator frozen 3 times. Once it rebooted automatically, once pressing Symb and On fixed the problem and once I was forced to use paperclip reset button. These 3 accidents occured when I was testing derivatives. I've got two questions:
1. Why Prime is not only unable to differentiate this function (I tested CAS too - nothing) but additinaly plotting this graph freeze calculator?
2. Is there any way to prevent such situations? Safemode?

PS. This is my first post and I'm not a native speaker, so I feel obliged to apologize for any mistakes I'll make