It is possible that the cable modem bridge (converter box to go from cable to ethernet) is getting overwelmed. Possibly by the virus mentioned. That box is on the Internet side of the firewall in your 2600 series router so it is getting hit with whatever traffic if floating around on your cable modem loop (traffic going to you and possibly everybody else on your city block). If that is the case there is not much you can do about it other than try to get your ISP to replace it with a different model. Is there a traffic lite on that box? Is it always lit up? Your sysadmin should be able to setup a packet sniffer on the Internet side of your firewall and see what kind of traffic is coming through and how much. If he is any good he should have done that already actually.

-Mike