Hey there ebbyambler,
I'm sorry to hear that your Ambient Mode settings are acting up! I can imagine this issue is pretty frustrating, so I'd be happy to help out. Now, are you following the instructions listed in this article? The instructions I'm referring to are these:
If so, let me know if the settings are resetting, or if they are still selected, and Ambient Mode on the Chromecast is not updating.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!
We haven't heard from you in a while so we’ll be locking this thread if there’s no update within 24 hours. If you're still experiencing problems, please feel free to start a new thread and we'll be happy to help.
Thanks for the help here, Jeran.