What to do when you believe your phone was hacked? I want to start fresh without buying a new phone I think my rooted cyanongenmod phone was hacked. What can I do to start fresh again? I already did a factory reset but after 1 month the phone started acting weird again (battery time dropped significantly, home screen mysteriously changes, unexpected reboots of the phone and the other day I saw that my phone was rebooting and installing something that looked weird at 3 am).

My phone is a Samsung Galaxy Nexus I9250 if that matters.

Should I format SD card also?
Reinstall stock ROM with odin with factory reset and install Avast Antivirus

A tutorial to reinstall stock here