My guess would be that that old rca wasnt shielded at all.. put the thing by a tv and see what happens.. I am amazed that Comcast didnt upgrade you to the Motorola Surfboard modems after the @home buy out..
right well it was AT&T that bought out @home in my area...they did nothing but cap my speed at 1500Kbps then Comcast bought my service and its been great..but neither company upgraded the modem ...I guess that would have been costly if done in a widespread way.