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I started RF1 again using Dash Faction 1.6.1, and every 5 - 6 minutes the framerate will drop to 10, sometimes even 5 causing stuttering/freezes. Has anyone else encountered this, and solved it?

GPU: RTX 3080
CPU: i9-9900K
I've not encountered this, but it is strange. In the Dash Faction launcher, there's a link near the bottom labeled "Discord Support Channel". You should click that, and you'll be directed to the #dashfaction-support channel on the Red Faction Community Discord, which would be the best place to ask about and get help with this (and any other issue you may encounter). The entire community is there, including Dash Faction's developer.
I did a fresh install with Dash Faction 1.6.1 and played for ~ 15 min. I had a super stable framerate of 200 fps without any crazy drops the whole time even with the AI enhanced texture pack installed on top.
I reinstalled both the game and Dash Faction and it seems to be working fine now. Played for about an hour and no drops.

Thanks for your help.