I switched the office over to the T1 that normally carries our VoIP traffic and it's been smooth sailing ever since. This is a Razorline connection.
traceroute www.google.com shows hops over level3 to 220.127.116.11. Over the Comcast link I was resolving www.google.com to 18.104.22.168 - which appears to be accessible now over the Razorline. Haven't attempted to access via Comcast, so it could just be that it's back up.