I just got my XFinity service setup on Saturday and overall have been very happy but began noticing my internet would drop for 120-150 seconds at a time at random times a day, sometimes 4-5 times per day. The local connectivity within my network is always solid during these outages so I knew it was something on the gateway itself or upstream from the gateway. I logged into the XB3 gateway and saw in the event logs the "erouter0" interface continuing to get a new IP (which happened to be the same one over and over) and every time it happened I saw the 120-150 second outage.
Here is the snippet of the logs from the XB3, as you can see no consistency in when it's happening.
[Dhcpc][20863]: erouter0 got new IP 98.220.186.202 Jun 4 11:27:25 Notice
[Dhcpc][529]: erouter0 got new IP 98.220.186.202 Jun 3 21:50:26 Notice
[Dhcpc][22401]: erouter0 got new IP 98.220.186.202 Jun 3 19:27:16 Notice
[Dhcpc][20949]: erouter0 got new IP 98.220.186.202 Jun 3 19:24:06 Notice
[Dhcpc][18252]: erouter0 got new IP 98.220.186.202 Jun 3 19:00:47 Notice
[Dhcpc][16206]: erouter0 got new IP 98.220.186.202 Jun 3 18:48:02 Notice
[Dhcpc][14703]: erouter0 got new IP 98.220.186.202 Jun 3 18:44:10 Notice
[Dhcpc][8747]: erouter0 got new IP 98.220.186.202 Jun 3 08:58:01 Notice
[Dhcpc][5214]: erouter0 got new IP 98.220.186.202 Jun 3 08:48:14 Notice
[Dhcpc][921]: erouter0 got new IP 98.220.186.202 Jun 2 14:50:46 Notice
[Dhcpc][23486]: erouter0 got new IP 98.220.186.202 Jun 2 12:35:08 Notice
[Dhcpc][18979]: erouter0 got new IP 98.220.186.202 Jun 2 11:45:45 Notice
[Dhcpc][17500]: erouter0 got new IP 98.220.186.202 Jun 2 11:41:24 Notice
[Dhcpc][15144]: erouter0 got new IP 98.220.186.202 Jun 2 11:24:19 Notice