You have an old Radio Shack VU-90, which, in it's best days, was a marginal performer. When you add the disadvantage of being installed in the attic to the mix, you have the potential for some trouble. Alignment would be my first suggestion, however, if your cable is 20 years old, then that would be my next guess. To find out what your proper alignment should be, use the TVFool.com station identifier here:
http://www.tvfool.com/index.php?opti...pper&Itemid=29
based on the San Dimas zip code, your correct alignment should be just about 290 degrees by the compass.
If you want to be sure that you get everything reliably, mount a new Winegard HD-7694P on the roof with all new cable.