fiber wired connection can't connect intermittently

The BGW320 has issues with the Ethernet LAN ports.  The worst one is the 5 Gbps (blue) port, but the yellows are known to have some issues even on "good" cables.  Some have seen improvements by changing to higher spec cables or inserting a Gigabit Ethernet Switch between the Gateway and the Ethernet-connected devices.

Ironic that Ethernet is less stable than Wi-Fi.

Can you confirm the firmware version in the gateway?  That’s just it about firmware, it can make a perfectly working gateway do strange and inconvenient things.  
One simple thing you can do yourself is to factory reset the gateway.  This can clean things up that have built up over time.  At some point if dealing with Att directly they’ll likely try to send you a refurbished replacement, but generally doing a factory reset accomplishes the same result. 
About the cables, I’ve also seen anecdotal evidence of greater sensitivity in the 320s, but no technical explanation has ever been brought forward to explain what requirements the 320 has in this area to make one particular cable work in lieu of others.  Perhaps adding an inline switch to isolate your home network wiring from the gateway itself might help.