If you are physically disconnecting the cable from the back of the TV and moving the connection to the tuner card, problem may be with the tuner card not being as good as the tuner in you TV. Does the tuner software give you the option of selecting cable or broadcast channel usage and is it set to broadcast.
EDIT: Just saw your update. Is the picture snowy or does it look like it is freezing and/or breaking up into blocks of pixels.
EDIT: I just checked on the PV-CX881P model number in your last post. It does not have any kind of tuner.
http://www.prolink-usa.com/