I recently purchased a Chromecast Ultra as an upgrade from my old Chromecast, and it works fine on all apps apart from Now TV. It plays for about 2 minutes and then the picture quality degrades to the point where it becomes unwatchable. If I then switch back to my old chromecast to play the same content it's fine.
There is no issues with my broadband and I've been through trouble shooting with support and nothing fixes the issue, just seem like there is a issue with the service on this device.
I have similar issues with my ultra. It doesn't degrade but I keep getting the spinning NowTV logo suggesting it's buffering but it never recovers and we have to stop/start it.
This is on two different ultras on two different tellys, no problems with Prime Video, BT Sport, Netflix or Stadia. Using the LG NowTV app on the TV works fine so it seems specific to NowTV and my ultras 😐