What is the maker and model of your tv?
Could you check if your tv is running at 30htz, 50htz or 60htz?
What is the resolution of your tv and what resolution you are running Stadia?
What device are you using to run Stadia?
Is that device connected direct to your tv or a home theater receiver?
That maybe could be a downscaling problem. But I am not sure yet.
Meanwhile you could try this:
If you are using the Chromecast, In your phone open the google app home. open your Chromecast there. Go to settings and which on the function "Use 50Hz HDMI mode" and check if gets better.
If you are plugging a windows computer to your TV, click with the right mouse button in your desktop and select
your graphic card settings. Intel, AMD, Nvidia, etc.
In the display settings try to change the refresh rate to 50htz or 60htz. Also sometimes there is an option to display your computer to a monitor or a tv. Try to play with this setting.
If you can not select 50htz or 60htz try to change your HDMI cable.
