I swapped cables, swapped the firewall for a white box pfSense, swapped the firewall with a UniFi USG, placed a switch in between the firewall and the ISP, and the interface was still flapping. The ISP is a WISP. They supposedly swapped out the radio on top of the roof, but it was still flapping. I wanted them to replace the cable and the PoE injector, but they didn't. We ended up hard setting the speed/duplex to 100 FDX and the connection is better, but still not perfect. The customer is going to be changing ISPs.