Performance issues, unable to identify the cause

Entering the main menu is a world of pain. Audio slows down to a choppy mess and the mouse hangs horribly. Checking my task mgr I notice huge CPU spikes. When I enter the options from the menu, the game runs perfectly. When there's a video playing, the game also runs great. When actually ingame, Ill have the performance issue return every minute or so. With periods of high fps in between. I'm truly mystified by this issue, and have tried a few things to fix it.

Interesting to note is that when I alt-tab out of the game, it runs fine. Since I can hear the audio in the background running fine, not choppy at all. I cant shake the feeling it is coming from the menu system.

Ill start with my specs:

Intel i5 2500k OC'd to 4ghz
Asrock P67 Extreme
8GB DDR3 1600mhz 
Gtx960

I realize the CPU is getting a bit old, but it is still working for the latest games. Is wfto that heavy on resources? According to the min specs I should be able to play this perfectly.

Things I have already tried:
First of all, I dont have integrated gfx.

I found an issue with it starting in dx9 instead of dx11. Managed to fix that but it basically made it a bit worse.
Also tried dx12 but no joy.
Re-installed my drivers (and drivers only) after a fresh uninstall with ddu.
Disabled my OC on my rig, but had no effect whatsoever.

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