Most likely low signal. Do what is siggested above.
Also, be aware that even though you have the cable connection for the modem, plugging in the cable into a TV or tuner is considered theft. If they ever find out, they can sue you for lost business revenue.
If you don't want to be sued, the same 13 channels you get now, are most likely free over the air channels.
6 TV's in the house on FiOS and we only pay $4.99/month to connect them all!!! Power to the CableCard and WMC7!!!