You've probably already done the basics - like check all the connections to see they are still tight and water has not gotten in.
After that, I'd try moving the antenna up or down a few inches. Sometimes you find a sweet spot.
And, I'd make more plots and post the links using these instructions:
http://forum.tvfool.com/showthread.php?t=14508 -- Since yours is at 12' try 15', 20' and 30'. The information could be very useful.
It's possible it's an issue at the transmission tower, but unlikely. People often complain and the engineers tend to find and fix the issues. You certainly call call the station engineer and ask.