Why is my Samsung Note 4 freezing?

[AC Question]

I suppose it all started after 5.1.1 update.
My note 4 repeatedly started to freeze completely. The screen is black, it does not respond in any way to any buttons, attaching the charger has no effect at all, only hard reset helps, but it freezes again. I factory reseted it few times, wiped cache partition many times. It freezes also in safe mode and shortly after fresh factory reset with no new apps installed. It also sometimes freezes after charging to 100% - again only hard reset brings it back to life.
Another issue is with the fingerprint scanner and forgetting all wifi passwords - it happens almost every day - after switching on it does not recognize my fingerprints and always forgets all wifi passwords, have to restart it, then it recongnizes fp but I have to reinstall my backuped passwords to wifi's from KIES.

What could be the reason for it?

The phone visited a service twice, always returned with freshly installed software and a not: all is working fine, but it didn't.
Now it is in the service again, but I doubt in their ability to repair it.

Any ideas?

 

[B. Diddy]

Welcome to Android Central! Sounds like it might be a hardware issue, if reinstalling the firmware didn't fix it. I'd start asking Samsung for a replacement phone, or at least a replacement of the motherboard.