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 126.96.36.199. Over the Comcast link I was resolving www.google.com to 188.8.131.52 - 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.