what is this?
[18:35:46] [ZWATCHDOG]: ZwiftWatchdog Fault on MainThread timer. Game state : Riding. Timer update delta of 62.5 sec exceeds timeout limit of 60.0. Abort is Disabled.
[18:35:46] [ZWATCHDOG]: ZwiftWatchdog Fault on Render timer. Game state : Riding. Timer update delta of 62.5 sec exceeds timeout limit of 60.0. Abort is Disabled.
The game got stuck for more than a minute for some reason. It’s not specific about the cause but perhaps some kind of CPU or GPU resource exhaustion or lock-up, which might be observable in Task Manager. When looking at CPU utilization you need to find the busiest single core since most of the CPU cores will be unused by Zwift.
Unfortunately I cannot access the taskmanager when that happens 
Merry Christmas
If you run the game in windowed mode you could, though that might possibly also make it not hang so you might not see the problem
By the way if running in windowed mode results in the game being stable, you could use Borderless Gaming to mimic full screen mode. Latest version in the Steam store: https://store.steampowered.com/app/388080/Borderless_Gaming/
I’m still running the last legacy version which works fine for me: https://legacy.borderlessgam.ing/BorderlessGaming9.6_admin_setup.exe
For people looking for an answer I think I have found the solution, increasing tdrdelay and tdrddidelay in the windows registry