Low fps on video games with 1080 graphics card

I have an issue with low fps in Fallout 4 (with minor mods that shouldn’t effect performance too much) and Rust which are so far, the only video games I've played recently. I never remember having this issue a few months back when playing Rust, CS:GO, The Forest, Stranded Deep and Skyrim (I modded Skyrim with heavy mods, such as enbs’, and played even without mods and they ran similar with only 5-10 fps hit occasionally in combat when using mods) though it never really got passed 40 fps, which I still find a bit of a let-down. Recently I have been getting around 20-10 fps in combat in both Rust and Fallout 4 affecting my gameplay and PvP greatly, though comparing Fallout 4 gameplay on my Xbox (which is using heavier performance impacting mods than my PC version of Fallout 4) is worse performance wise compared to my PC gameplay. Usually these two games run at about 25-30 fps on my PC without combat and away from built up areas in game, but these games are full of combat so it’s quite rare to get that much fps (I do get 40fps when I stand very still and look at the floor). Judging from this I doubt it’s due to lag caused by poor internet. I don't think bottle necking is the issue either as when playing Rust I got around 53% CPU and 32% GPU utilisation using HWMonitor and the GPU utilization sometime exceeds CPU and normally levels with one another, however task manger was giving me lower figures: 11% CPU and 31% GPU making me not sure which readings to go with. I've also changed the Vsync on Nvidia control panel to ‘adaptive’ from their default option 'Use the 3D application setting', as Nvidia suggested to do, that did nothing and made everything shaky and my screen split and looked dicey but I noticed a slight fps raise of about 5 fps when in Diamond City in Fallout 4 (from around 20-25 fps to 25-30 fps). I did overclock my CPU to 4800 Mhz from 4200 Mhz to experiment and other lower values in the 4400 range I did see some performance gain of about 5 or 10 fps if I was lucky but not much. I have permanently made graphics lower quality in my games, which I wanted to avoid, and I didn’t see a difference in Fallout 4 but had about a 5 fps gain in Rust. I also sometimes have random dips in fps say when I reach 30 then randomly dips to 10 or 20 fps in most games, this has always been an issue though has been occurring more frequently ever since I built my PC a year ago. I have set my fps cap to 120 as my monitor has 60hertz refresh rate. Sims 4 seems to be the only game that runs decent at around 40 fps on the best graphics and over 2000 mods, it does have some major dips for a few seconds to about 10 fps (which annoys me as it’s my least favourite game I own). I have updated my GPU Drivers which managed to lower my total score when using Valley Benchmark and attempted to optimise my CPU and GPU for gaming. These are all the things and methods I remember trying. I do not know what to do after most having little to no effect and I hope it’s something I’ve stupidly missed.

My specs:

GPU: EVGA GeForce GTX 1080
CPU: Intel Core i7-6700K 4 GHz
OS: Windows 10 Pro 64 bit
RAM: Corsair CMU16GX4M2C3000C15 Vengeance LED 4x8GB
MOTHERBOARD: ASUS MAXIMUS VIII HERO ATX
MONITOR: Acer Predator XB281HK 28-inch Gaming Monitor 60Hz refresh rate

Thank you for reading. Any help and time spent trying to resolve this issue will be greatly appreciated.