If it's analog terrestrial/cable tv then it might be dozen things starting from poor singnal, power lines around the signal cable, poor quality of cable.
Analog TV Tuner cards have much higher sensitivity compared to normal TVs.
You can try connecting the card through VCR (with build in tv tuner), coaxial cable to VCR then S-Video/SVHS cable to tuner card.
Digital signal is way less sensitive to above behaviors and it delivers way better quality and sharpness.
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