Unstable FPS in Games - Such as Far Cry 5, Witcher 3, etc.

Hi,

Around Christmas time I upgraded from a GTX 970 Graphics card to a GTX 1080ti, and I've found that my performance gain has been quite minor, with there being hitching and much lower fps than I was expecting in certain games.

For example, I recently bought Far Cry 5 and have been playing through it, and although the game has been using only 3-4GB (according to the game's menus) of the 1080ti's VRAM, the game sits around 40-50fps with occasional increases into the 60 area when inside a building.

I've also found this to be a similar case with other games, such as Star Wars: Battlefront 2 (2017),The Witcher 3, Black Desert Online, Divinity: Original Sin 2, to name a few. In these games I can hit 60, but its only occassionally while in open spaces. As soon as I enter an area (such as a city) the fps plummets to 30-40 fps. This would be fine, if I could just lower the settings to account for this loss, but doing that does nothing.

I've looked online and attempted a couple of fixes, namely:

-Ensuring GPU Drivers are up to date,
-Changing settings in the NVIDIA Control Panel, i.e. Resolution Scaling, Max Pre-rendered frames, Power management.
-Switching Windows 10 to Gaming Mode.

I'm not the most tech literate, so please bear with me if its something obvious!

I've included my full spec list down below, but if anything else is needed, just ask and I'll do the best of my ability to give it to you! Also, if I've posted too much of my spec, tell me and I'll trim it down to the necessities! Thanks in advance for your help!

My Specs:

-OS Name Microsoft Windows 10 Home
-Version 10.0.16299 Build 16299
-Other OS Description Not Available
-OS Manufacturer Microsoft Corporation
-System Name AndyPC
-System Manufacturer MSI
-System Model MS-7885
-System Type x64-based PC
-System SKU To be filled by O.E.M.
-Processor Intel(R) Core(TM) i7-5820K CPU @ 3.30GHz, 3301 Mhz, 6 Core(s), 12 Logical Processor(s)
-BIOS Version/Date American Megatrends Inc. P.00, 20/03/2015
-SMBIOS Version 2.8
-Embedded Controller Version 255.255
-BIOS Mode UEFI
-BaseBoard Manufacturer MSI
-BaseBoard Model Not Available
-BaseBoard Name Base Board
-Platform Role Desktop
-Secure Boot State Off
-PCR7 Configuration Binding Not Possible
-Windows Directory C:\WINDOWS
-System Directory C:\WINDOWS\system32
-Boot Device \Device\HarddiskVolume2
-Locale United Kingdom
-Hardware Abstraction Layer Version = "10.0.16299.98"
-Username AndyPC\Andy
-Time Zone GMT Summer Time
-Installed Physical Memory (RAM) 16.0 GB *Total Physical Memory 15.9 GB
-Available Physical Memory 9.22 GB
-Total Virtual Memory 24.8 GB
-Available Virtual Memory 14.0 GB
-Page File Space 8.89 GB
-Page File C:\pagefile.sys
-Device Encryption Support Reasons for failed automatic device encryption: TPM is not usable, PCR7 binding is not supported, Hardware Security Test Interface failed and the device is not InstantGo, Un-allowed DMA-capable bus/device(s) detected, Disabled by policy, TPM is not usable
-Hyper-V - VM Monitor Mode Extensions Yes
-Hyper-V - Second Level Address Translation Extensions Yes
-Hyper-V - Virtualisation Enabled in Firmware Yes
-Hyper-V - Data Execution Protection Yes

I also have 2 1920x1080 monitors which are connected to my PC.

Thanks in advance for your help!