Sounds like an issue with the CEC computer in the TV recognizing the Arc, and then unrecognizing it, over and over. I’d double check the TV for firmware updates, apply them, then actually reboot the TV by unplugging it from the wall for five minutes. Just turning it off with the remote or even a button on the TV  won’t do it. 

If that doesn’t work, I’d reboot the Arc, and double check to reseat all the HDMI cables. It could be something as simple as the cables not making constant contact.

Finally, if it’s still occurring, I’d submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.