My guess, given the things you’ve already tried, is enough wifi interference to impact the higher bitrate of a line in. I’d certainly look for potential issues, both inside your home, and effects from outside, too.
If all else fails, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.
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 Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.
If nothing is attached to PORT’s Line-Out, there is no reason to Group the two units, just play the Line-In on BEAM.
Or set the Beam as preferred speaker for the Port.
