Check around the Arc (not ARC) for sources of 5 GHz wifi interference.

Then refresh your Sonos network by unplugging all Sonos devices, and rebooting your router. Only when the router comes back up should you plug the Sonos devices in to power. 

The above option can be skipped if you choose the more permanent step of assigning reserved IP addresses for your network devices, including all Sonos devices, in your riuter’s DHCP table. See your router’s manual on how to take that step, each router manufacturer does it differently. 

If none of those fix the issue, I would recommend that you submit a system diagnostic within 10 minutes of having the surrounds drop out, and call Sonos Support to discuss it, or post the number here.

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, there are more options available beyond just the diagnostic analysis.