I went with 2160p first off with MSAAX8, have tried all kinds of settings.
Game fluctuates rapidly. Here all the cars have gone and it's still going from 33ms up to 100ms. This is with the CPU affinity set as well. Without it my CPU looks like yours maxed out on one core. Either way though the stutter is the same.
What I have noticed is when the game launches in window mode is seems smooth and good frametime and such (I know desktop will shift to 60hz), soon as I go full screen all hell breaks loose. The menus and the game are spiking like crazy.
It's not a good sign with your CPU that one core is still maxed out after all the shit with FH3 over 30fps. Even people with the latest CPUs at the time and DDR4 clocked to 3000mhz had sutters over 30fps. That one core pegged to max is the same we had in FH3.