I would suspect crappy connectors first. The cable company should fix the problem. I have transmitted 100 watts of rf just inches from the modem with no problems. I have seen a wireless computer router wreak havoc with a cable modem. Those modems can be sensitive to stray rf. If the problem persists with proper connector changes, then you can make a filter for pennies provided you know the frequency the modem operates. These cheap class 15 gadgets don't like ham radios some times. If you don't have time you can pick up a filter at Wally World for about $15.