You're a star thanks.
The issue is present on mine from the main menu of the game. At the main menu there is a map that spins around and about every 10 secs it pauses for a second, drops from 300fps down to 1fps when running at 800x600 windowed.
In game actual gameplay is the same, frame drops in a similar way where i be at uber low res / settings or 4k high settings, it's the same behaviour.
Funnily enough this is my boys game and on their laptop with i5 gpus it runs smoothly.
Thanks again, very interested in your results!
I have managed to have a quick try.
left it in window mode on the desktop for a bit and walked away
hope this helps, spcs below
I7 950 @ 3.06ghz (no OC)
4GB DDR 3 1600mhz ram
390 8GB
this was using Afterburner, set it to use 1102mhz core and seems quite stable