I'm only reviving this thread in case some network guru's have any thoughts. I'm 99.9% convinced its an ISP issue though.
I created a new thread in the comcast section here at DSL reports to see if anyone over there has any ideas.
I wrote a batch file that I could quickly run when the issue comes up, My batch file pings my modem (on the WAN side of the USG50) and then pings the ISP gateway (my first hop). When the issue came up, I COULD ping the modem (proving traffic is moving from LAN to WAN and back on the router), but the gateway timed out. Once the issue cleared the gateway started responding to pings again.
For more info (in case you can help) my new thread is here:
»momentary internet issues-help.