Check for water in connectors. If moisture get's into the connector it can travel a long way into the coax. If that happens, odd symptoms crop up, and may seem to change over time.
Also, look for wear or corrosion to the antenna and balun (matching transformer).
If you have an amplifier in the system, a nearby lightning strike or static discharge could have damaged it... A power surge can do so also.
Perhaps the easiest way to start is to perform this test using a new cable.
http://forum.tvfool.com/showthread.php?t=13646