I contacted NVIDIA tech support and they checked out my system and couldn't find anything wrong. However they recommended I change my 3D settings (GTX1080) as follows:

a. Power management mode - Prefer maximum performance
b. Vertical sync- Off
c. Threaded optimization - off
d. Triple buffering - off

The game runs smooth BUT after about 15 mins (sometimes 25 mins) I get a sudden jerk/stutter and either the game freezes/crashes to desktop straight after, or I might get a few more minutes of gameplay before it happens. There is usually a Ubisoft pop-up "crash report" or a windows pop-up saying "GRW not responding...".

I've been working with Ubisoft Support for nearly 3 weeks now, and they are stumped. Game itself, when it's not freezing/crashing, is a lot of fun to play and I really feel miffed that I can't progress on it.