Random FPS drop in Alliance Raids

[Final-Fantasy]

You could try connecting your PC to your Phone, by using the Phone as a Hotspot, most have this feature in the Network settings.
Doing so you can rule out a connection issue.

It maybe in fact an Audio issue, you could try Muting all of the Music and Sound effects, I'm not actually sure if that would matter as its just Muting so its probably still processing the Audio.

If you have multiple Audio devices, such as Nvidia HD Audio and 3rdparty PC Audio apps, you might want to tinker with those, as since you can do some Raids fine, it could be Audio related, as you mention you get FPS and Audio stutter.

Another thing you can try is completely remove Nvidia Geforce Experience app, but first, disable Shadowplay and also turnoff Battery Boost and Nvidia Overlay, all of these options are in the GFE app at the top right (little Cog).
If turning Shadowplay, Battery Boost and Nvidia Overlay off do nothing to help, I would uninstall GFE and then test, as GFE and those apps in it are known to cause serious stutter issues in certain games.

If you do remove GFE, you can get or update your Graphics card driver at the Nvidia site, and only choose to download the Driver by itself, not the GFE app, you don't actually need GFE at all.

I would give that a try and see if it fixes the issue.

[ejunniim]

I would give that a try and see if it fixes the issue.

Somewhat late answer, but I tried each of these ways, just to be on the safe side!
My connection is still stable, and sound changing didn't help. Also reinstalled nvidia drivers, with and without GFE. Alas
I guess it is possible that the answer we need is hidden somewhere in tech info that I provided, but, sadly, I myself am not an expert and cannot understand whether this is true or not. In an extreme - extreme - case, I can try to reinstall the system itself. But that prospect sounds terrible, lol. After all, have been working with this computer for many years...

[ejunniim]

Well, since tech support hasn't been received and the thread has finally died down, it might as well be closed.
Thanks to the user above for the effort to help.
Will add that I had to reinstall Win10. It helped.