The problem has been identified, although not resolved. I have found a solution that appears to be working for the moment.
Cablemas appears to be sending an invalid IP address to our cable modem. Web surfing still works, but more complicated services like voice traffic and VPNs that involve two-way communication were down for the count because of this.
This is the 3rd time in the past 2 years that this problem has caused me grief. This is frustrating to me, and I work in the industry. I feel sorry for the average person who has these types of problems and tries to get Cablemas to admit they are the cause of the problem. Every time I call to explain the problem to them they act like there is nothing wrong. I persist, and eventually they resolve it. Unfortunately, they don't compensate me for my time or even say they are sorry for the problem they created.
As I mentioned, there is a work-around until Cablemas can fix the problem on their end. If anyone is interested, please PM me for details.