Arris CM8200A intermittent problem

A few months ago my ancient modem was upgraded by Cox to an Arris CM8200A. Ever since I've had intermittent problems with the connection becoming very flaky, with very high packet loss until eventually nothing gets through. Sometimes I'm good for weeks, sometimes the problem recurs within days.

The "solution" which works 100% of the time is to reset the modem. After the reset completes everything is completely fine until the problem inevitably comes back days or weeks later.

Another 100% reproducable aspect of the problem is that once it starts happening, I can no longer access the web interface on the modem, or even ping it from within the local network.

Here's a graph of ping times from inside my LAN to the modem, and to the main webserver at www.ucsd.edu which is - at least physically - somewhat close to me. I also run this graph on Google's DNS server with similar results. As you can see on July 28th around 4:00am an instance of the problem started which then lasted until I reset the modem just before noon. During the problem ping statistics to the modem were 100% packet loss, but the stats to the UCSD server were about 60% packet loss. So the connection to the internet is not totally dead...

Is this a bad modem, bad firmware?