PCX1000 Interesting Problem

Here is the problem....

I have three computers connected via a netgear router.

Computer 1 is Windows 98 (no antivirus software)

Computer 2 XP Home

Computer 3 XP Professional

I had this cable modem for about a year with no problems what so ever. Then one day I can't play a simple pc online game without some serious lag that happens randomly. I decided to check out the wire since it is located on the side of my house(i know this is not a good idea but it works and this is not the problem as i will elaborate further). I connected the 98 machine to my cable modem directly(bypassing wire outside as well as the netgear router)and pinged google as well as the router that the cable modem connects to. Random request timeouts occurr so i called my ISP. After some troubleshooting he tells me that i need to replace the cable modem because the mac address of one of my PC's is buried in the memory of the cable modem. The reason being is that all computers were disconnected from the cable modem and the mac for this particular computer(win 98) was still showing on the cable modem that it was attached. The tech guy tried erasing all attached devices but it did not work. My question is it possible that someone hacked my cable modem and bypassed my router to steal bandwith? Is the cable modem a simple piece of garbage?

I would like to hear from anyone about this as i am curious to know if others have experienced this and if so under what conditions. If you have any idea as to what happened here please don't hesitate as i plan to get a new modem and don't want this to occur again(i.e. if it is a virus). Preferably, I would like to keep the cable modem but it doesn't sound like a good idea. I would gladly take any suggestions

I spoke to a few friends and they notified me that it is possible for a MAC address to be embedded into a cable modem. Time for a new modem

I guess I should start placing more confidence into tech support for Road Runner