Rytis2

I have almost the same internet speed as you and my ping is even better at 1 ms. And in the years I've been playing GTA 5 Online, I get the same issue when lobbies are more populated than others. Here's a couple of videos I made last month showing exactly what we are experiencing...
As you can see, my fps is much higher when in a solo lobby when compared to a populated one. 82-103 in a solo lobby, 68-80 in a populated one. And this is just when in this area of the game. Inside city areas, sometimes I've seen my fps creep into the 50's and even into the high 40's a couple of times. When that happens, it is super noticeable. It's just how GTA 5 runs on Rockstar servers when more people are in the lobby. I cope with it by always moving.
 
I'm unsure of the exact reason this is happening. I've thought of a need of higher RAM speed, higher internet speeds(especially upload), other peoples internet speeds and if they ALL need to be great to get higher fps. Not really sure of the culprit and it's tough to test these things.
 
There's also the thought of more people in the game world and that in turn could hit fps in some way, I dunno. I mean, they are off doing god knows what in the game world. Maybe the data stream affects fps. What I do know is that this is a common issue with a lot of GTA 5 Online gamers. Everyone seems to have different theories of why it's happening.

Mobo is ok, ram is meh. Amd performs better than intel in most tasks currently.

Xilence Performance A+ and X (preferred) are decent.
 
be quiet Pure Power 10 and 11 with 400W and more are also recommendable and have better fans than the Xilence.
And they aren't even that expensive.
 
So you have some good options in that shop.

I have this PSU, so far it works great (have it like for a year). It has a premium look and feel to it, and also utilize modern technology (DC-DC conversion, rather then multiple transformator inside).
Yeah, 6+2 pin means, that it is basically an 8 pin, but it's divided in a 6+2 manner, so it can also fit 6 pin.
It has even 2 6+2pin connector, so it should be enough for upgrades.
 
Sparks are not a good sign, in any place inside a computer... That was a heavy warning for you!
You know, these kind of cheap PSU can even cause fire, so it's a potential risk. Always go for a quality PSU and MOBO, that's rule #1.  You can save money on fancy cooling and rgb  not on these parts.