Cable modem firmware bug?

Our organization has been having to power cycle our Comcast-provided SMC cable modem at least once a week.  Comcast has replaced the cable modem (with the same model), replaced the cable fittings, jiggered the power transmission levels to/from the amplifier, etc.  Nothing seems to prevent the users from reporting (first) that Internet access gets slower and (second) loss of external connectivity.

From outside our facility I can ping the external IP address of the cable modem (.74) , but the "internal" static IP address assigned to us by Comcast (.73) doesn't respond.  From inside the network attempting to hit the management web interface of the modem (10.1.10.1) fails with "Server not available." Internal network resources are all available, but if DNS needs resolution by an outside domain name server, that connectivity isn't available and so the attempt fails.

I can't think of anything else that could be causing the cable modem to be addressable externally but not internally.  Well, except for people having disconnected it from our network!  But since power cycling fixes the issue, I know that no one made a physical break between the cable modem and our internal network.

I have at least once in the past had a VNC connection open from home to one of the computers inside the facility.  My connection would stay open and I could use the computer on inside (via VNC) to see that new outbound TCP connection requests to the cable modem were going unanswered.  Eventually, even my VNC connection would drop.

It sure feels like an SMC firmware bug is being encountered, but I fully expect our network to be an example of a really simple network such that if there is a bug many more customers would be hitting it.

Nonetheless, we're pretty frustrated by this continuing problem so I'm will to see if anyone has any thoughts.  Thanks!