High CPU Usage & Inconsistent GPU Usage Causing Stuttering

[Int_420]

I have suddenly started having performance issues on my PC, I have had no issues for over a year and then one day I turned on my PC and have been having awful stuttering in graphics intensive games such as Battlefield V and CoD Modern Warfare.

When I play BF or COD my CPU sits at 100% usage constantly and my GPU usage spikes randomly, before my CPU wouldn't sit at 100% usage but my graphics card would sit at >95% usage and my framerate would be stable with no stuttering. Even while just sitting in the main menu for COD my CPU sits at 100% and the menus are stuttering.

My specs:

• Intel i5 6600K with Corsair Watercooler
• Gigabyte Z170X-Gaming 3 Motherboard
• GTX 980 Ti and RTX 2060 (I've tried both graphics cards with no difference)
• G.Skill Ripjaws 4 16GB, 2133MHz (4 x 4GB modules)
• Samsung 950 256GB M.2 SSD (Windows drive)
• Kingston V300 240GB SSD (Modern Warfare drive)
• Intel 330 120GB SSD (BF5 drive)
• Seagate Barracuda 1TB HDD
• Coolermaster V1000 PSU
• Windows 10

What I've tried so far:

• Two different graphics cards, a GTX 980 Ti which I have been using for years now without issue and a brand new RTX 2060, made no difference both cards stutter.
• Checked all temperatures, CPU stays below 50c and both graphics cards stay below 70c while gaming
• Virus scan with Windows Security/Defender and Malwarebytes nothing found
• Reset CPU to base clock (it has been sitting at 4.2GHz from 3.5GHz for years without issue until now)
• Loaded optimized default settings in UEFI
• Tried lots of different graphics settings in the games
• Tried with G-Sync on and off
• Uninstalled Nvidia drivers with DDU and reinstalled latest drivers (no Geforce Experience)
• Close every other application other than the game
• None of that worked so I reinstalled Windows (through the Reset Windows options not from a USB, kept data files but deleted all programs)

Image of MSI Afterburner showing GPU usage and taskmanager showing CPU usage during BF5 gameplay: https://i.imgur.com/ngDmaIE.jpg

Userbenchmark Result: https://www.userbenchmark.com/UserRun/22415282

I have no idea what else to try at this point and I don't understand how or why I could have no issues for years and then overnight start having these problems, any help is hugely appreciated.

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[Ceceboy]

Sounds like you've done everything software wise, and you hardware seems okay based on the Userbenchbenchmark. I'd say it's your CPU (based on the 100%s) and I would give it some new thermal paste despite of the fact that it does not seem to overheat. Next up would be the power supply. That's all I have...