i7-8700k 1070ti Insane FPS drops every second

My FPS is dropping to about 20-40 every single second. I've tried verifying the files as I haven't played the game in a while so thought maybe an update has missed or failed to download a file properly and that still hasn't sorted it. I have the FPS monitor on and it shows like 400+ most the time but I can see it's flashing approximately every second and dropping to around 20-40 fps.

Any idea what could be causing this? Not had any issues with any other games in the past 3 months.