Galaxy Note 4 virus?

[PoptartHunter0]

So my Galaxy Note 4 has been acting really weird for the past 2 weeks.
Here's the problems it has:

I have full brightness and the brightness sometimes lows and then goes normal again randomly and no auto is not checked.

The phone volume sometimes goes really low when it's at max and I have to turn off the screen and works again.

Apps crash a lot, youtube crashes a lot, so does twitch and other apps and I have to turn off the screen and it also takes a lot time.

And finally. When I open Web browser sometimes a new tab opens saying "your phone has a virus, download this now" and other weird adds which make me think I have a malware on my phone.

What can I do, what do you think it is, yes I had to admit I have downloaded some stuff that I am not sure about, so any help? Should I reboot it?

I downloaded a lot of antivirus apps and none detect anything, my phone has never been hit or have any damage.

Help

 

[D13H4RD2L1V3]

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