What seems to be the problem with my new lenovo s820?

I just bought a lenovo s820 last 2 months now the phone has gotten slow I though it has caught a virus so I had to factory reset it

but it is getting slow again and especially when I'm playing angry birds go it suddenly closes. Also tetris and another thing that happened is when I open the browser and also google chrome it also suddenly closes. This time instead of factory reseting it again I just powered off, removed the battery, put it back then powered it on again. It is working fine but I'm still worrying what if it happens again?

I mean what seems to be the problem of this phone why does it suddenly becomes slow and suddenly closes apps? I scanned it before with the built in norton mobile but it says no virus has been detected. I also haven't installed apps outside of what I download from the google play.

Oh no. That's Cerberus, guardian of the Underworld! Well… Sort of. Something's wrong with him!