Hi there @mrbalaclava, I'm really sorry to hear that this is happening when you play AC Valhalla.
It sounds like you have already done a few of the steps that I was going to suggest, updating your GPU drivers and verifying the game files.
From here I recommend doing a complete reinstallation of the game. We always recommend that your Ubisoft Connect client as well as your game are installed on the same drive that Windows is installed on (Usually C drive).
You can do the other troubleshooting that is recommended in our ...