Random things happening in my phone. What to do?

[AC Question]

I have a Samsung Galaxy grand phone and from last one week it's giving me a lot of pain. It is doing stuff by it's own. Whatsapp is running automatically, messages and even whatsapp calls are made automatically to random contacts in the list, outgoing calls are placed and even messages too.
Yesterday my wifi also got turned on and off itself. I don't know how to solve as factory reset the settings twice and installed everything again with an antivirus. But nothing shows up and my problem is still there. Is there anyone who can help me.

Thanks in advance.