I think i know where the issue is, did some testing and realized that this stuttering happens whenever the client has heavy interaction with the server, the frames take a huge dip.
How did i realise this? simple, when i start a local game, the frames are way more stable than when i am in a pvp server, and even on a local server you realise that whenever there is too much communication between the server and the client (e.g. when there is a respawn wave, or when a airstrike is called, also happens when there are many firefight happening at the same time in close proximity, etc.)

My specs are as follows:
I7 7700k 4.2ghz
RX460 4GB
16GB RAM DDR3 1600mhz
Common Samdisk HDD

Well thats what i found out, didn't use a presise methodology for those tests, but i hope that helps.
PS: sorry for any typos, english is not my first language.

You have to take into account that an online connection indeed requires resources and CPU time, especially when there's more than one person playing (which also means more bots in coop). I mean, multiplayer means to have more stuff happening at the same time, so this is kind of obvious and has been observed in many games.

In regard to your system - the CPU might be enough, but the HDD and low end GPU can mean trouble when the game loads more data during gameplay. I'm not at all surprised you're having problems with that setup.

Yeah i know that my specs arent exactly high end, but the game runs fine when those things arent happening, running at 60 fps, and most ppl dont have high end rigs, even more in my country, where a high end GPU can cost 1000$ alone.
Just saying "your GPU is bad bro", wont change the fact that even high end rigs run the game poorly, and that may be a cause, beacause even PUBG, which is a way bigger game, both interms of player count and map scale, than this one, runs with more stable fps at 1080p, while sandstorm has very unstable fps, meaning that i get 60 fps, but i have huge stutters even at 900p, which shows bad optimization.

I just tried to help, because many ppl are not playing this because of the bad optimisation, even though the game is great otherwise.

Well I get stuttering or frame tearing or whatever even in local games so its not just a server interaction.

The only additional performance issue on servers is that I'll get random rubberbanding even though my ping is low.