Been through Ubisoft support, an easy to forget, a simple installing latest video drivers and clean install fixed all my issues, try it.
Glad everything okay for youI had also contacted NVIDIA and did a clean install of my drivers. Alas, didn't solve the freezing/crash/PC reboot. However, have had an email from Ubisoft support with some stuff to try and working through this. One thing they suggested is to delete the GameSettings.ini file. However I don't have that file but do have a GRW.ini file which I deleted and the game creates a new one. So currently testing this to see if I can last longer than 15 mins.
I may have spoken too soon as the new patch 3.5 sent me back to crashing again! I have verified the files and even clean installed the latest drivers, about to give up on GRW much as I like it but the frustration is not worth it considering I have other new games waiting for my attention.