XG with two RED 50, one keeps disconnecting

We have an XG 230 with two RED 50s connecting branch offices. One site has been totally stable while the other site's RED 50 disconnects and won't pull dhcp addresses from the XG after it reconnects unless we restart dhcp on the XG firewall. I'm not sure whether it's a defective RED or a flaky ISP connection. Since the two REDs are the same age and use the same XG I'm inclined to blame Comcast. 

Does anybody have advice on troubleshooting to isolate the issue to the RED or the ISP? Also what should I grep for in the RED log to identify disconnects? I did it once with a support tech, but lost my notes