If you have problems with WiFi only and BT works well, it's driver related. WiFi fix required.
If both WiFi and BT don't operate, and you are sure all the fixes are installed, you have to unscrew all the screws under the back cover and tighten back. Sometimes it helps, sometimes not.
I'm having those problems once a week. Sometimes it's enough to turn the device off, shake it well, really well and turn on.
The WiFi/BT chip is badly developed in S3/Note 2 platform. The connector ribbon makes bad contact.