Galaxy S6 Edge Plus stutters and Youtube freezes while the sound still plays.

[AC Question]

Hello to all! Recently I bought an used Samsung S6 Edge +, it have Marshmallow Android; the phone as far as I can see is original and have no troubles in general and is not Rooted, however there are 3 things I am having troubles with:

1.-The Phone is fast however between menus I can see how it sometimes freezes for a second and then continues, something don´t even happen on my old Galaxy S4.

2.-Youtube plays for few seconds, then freezes with the sound still playing; sometimes I have to go back or forth on the video for it to resume, sometimes it resumes by itself after a couple of seconds but with frame and audio desync and sometimes it just says "An error has occured" so I have to tap for it to resume.

3.-Every time I turn off my Phone it forgets the password for my Wi Fi, something is VERY annoying and it have no option for remembering as far as I can see.

Now I already tested several options but nothing works so far:

1.-I already deleted Youtube app, reinstaled it, used the stock app, the latest one, grantes storage access to the app, tested from inside a Page in Chrome, etc.. nothing works.

2.-I already turned on Developer´s Options and enabled and disabled (tried several setups) "Force GPU Acceleration with Hardware" and these other "disable layers" options, and it doesn´t works.

3.-I already closed all apps in the background, wiped partition cache, and even did a Factory Reset, used the phone in Safe Mode, disabled the Ant+ and S Health, Apps, nothing worked.

I am really annoyed by this and so far nothing seems to fix it, I am about to discard the phone but I don´t want to as it looks really great in comparison with my S4, but with these faults I find hard to keep it, specially when my old Phone can do everything without any problem but a High End phone can´t apparently.

Any advice will be great, please!!

 

[Rukbat]

Start the phone in safe mode:

Power off the phone.
Wait one or two minutes.
Press and hold volume down button AND the power button.
Hold the volume down button until you get the lock screen.

Now all of your installed apps are not running. If that clears it up, load apps one at a time until you find the one that causes the stutter and either turn it off or uninstall it.

If that doesn't clear it up, you may have to clear the operating system's cache.

Power off the phone.
Wait one or two minutes.
Press and hold volume up button, the home button AND the power button.

Use the volume buttons to go up and down the menu, and the power button to select the current item. Wipe only the cache.