Well, further to my issues from yesterday of the GPU not getting 100% usage and the CPU dropping clocks and stuttering and what not. I installed the 2nd cable from the PSU to the GPU, so it has two cables now, instead of a single split one. I also got Corsair iCUE up and running to set the fan profiles on my H100i Pro cooler to something a bit stronger.
Loaded up AC:O again and ran the benchmarks several times with different settings and then just loaded up the game with my normal settings (Ultra High and both 100% res and 140% res). Same thing as yesterday. GPU never breaks 96% usage (despite testing with max settings and 4k/5k resolution) and the CPU still drops clocks and stutters like crazy.
Here's my Afterburner graphs from the session and you can see the CPU usage jumping all over the place and the clocks just going up and down. Everytime they go down, fits with the stutters I was getting in the game. Same thing happened with GTA V but not as severe.
EDIT: Forgot to mention, I also tried disabling MCE in the BIOS. It didn't make a difference at all.