This article contains information on signal issues with instruction on how to correct or work around these problems.
Low Signal Level:
If low signal levels are evident on all channels, double check cable/antenna connections and remove any unnecessary splitter devices. If the problem persists, add a "powered external amplifier with 5-10dB of gain" (consult your local electronics store).
High Signal Level:
If high signal levels are evident on some or all channels, remove any amplifier devices or reduce their gain. The addition of an amplifier or active amplifier/splitter with high gain to the TV cable input causes excessive noise on the signal resulting in distortion in the TV picture. If the problem persists, add a series "RF attenuator" (consult your local electronics store).