To cut a very long and painful story short, I had to start again with my computer i.e. fresh windows install along with fresh installs of everything else.
I re-installed STMR and for some strange reason I now get frame losses in places I never did before. The biggest losses are when viewing the map, if I'm parked up and open the map I go from 60 to 49 instantly. Its strange that its only when viewing the map it takes a sudden dip and stays at that fps. I used to always get a solid 60FPS on any map I've played (I generally don't use maps with 3rd party models) with any truck be it stock or mod, maybe the occasional 58 or 59 for a split second but not for prolonged periods of time. This was also the case when I was using Dynamic Weather mod, which I haven't tested since re-installing everything yet.
What I am a bit confused about is why this wasn't an issue before, but is now, and if anything with a fresh up-to-date version of windows, surely it should run even better? I am curious to know if anyone else has experienced something like this or know of a fix?
From all information obtained from the various monitoring applications available (detailed below), I have come to the conclusion that this may have something to do with the game not the system, however I believe that the newest version of Windows could also be doing something that I need to turn off or edit to allow STMR to run smoothly. I do vaguely remember seeing on a forum for a different game that some people had experienced issues with the Fall Creator update, but I can't remember if there was a fix or workaround, or what topic it was posted in on said forum.
Additonal information and tests:
Game is the current version
Uninstalled and then reinstalled the game
Using MSI AfterBurner, ASUS AI Suite and Windows Task Manager I can see that my machine is not even close to being stressed, i.e. barely reaching 40% usage on my GPU, CPU, HD and RAM as well as all temperatures are operating at the correct values.
I've gone through and reduced texture sizes of modified textures to be the same if not smaller than vanilla texture files sizes, even reducing the vanilla ones in places to see if that would help to no avail.
Using the 4GB patch
ReShade is active, but even with it turned off it makes no difference
Unzipped game folders and ammended config file accordingly.
Used Nvidia's control panel to modify the games 3D performance (the same as before the reset). The only setting I use is the VSync and pre rendered frames set to 4.
Not using STMod or any other hack
2 x GTX1080Ti in SLi
Game installed on a Samsung EVO 850 SSD
Seperate Samsung EVO 840 SSD System Drive
Playing on a 43inch screen in 4K, but if I play at 1080 the drops still occur.
p.s. I'm not a usually bothered by what FPS games runs at, but inconsistent frame rates get a little jarring after a while especially when there was no problem before with the exact same system specs bar the OS.