That is weird. I am playing on gtx1660, but also have 2 gtx1050ti plugged in the motherboard.
That additionally hampers performance. All video outputs are limited to 60fps, due to a mishmash of variety of screen models and I like to take fallout4 for a spin here and there.

I get stable 60fps on sea and dips into 40fps while on land.

I would suggest to download MSI afterburner or any other program you prefer to monitor temperatures and if the performance loss is due to thermal throttling. There's also a possibility to adjust fan curve there to a more agressive one, or just go straight to 100% speed. Do make sure to check your case aswell, have more fans blowing air in, than out, your PSU will be thankful too, if it's sucking air out from the case.

I do remember one time, this game has overridden my settings and went full blast. Closing the game, re-setting the adjustments in control panel worked for me.
Even if you just use "Adjust image settings with a preview" and move the slider either to performance or quality, should do the trick.

Double edit: What mods are you running?