[Tigerbangs]

You are experiencing a problem called multipath, where your antenna is receiving a signal reflected by the structure of the house and direct signal from the transmitter almost simultaneously: it's a very common problem with attic and indoor-mounted antennas. Because the wavelength of TV signals are different according to the transmitting frequency, or channel, the problem is often selective, and might not be seen on all of your channels. A multi-directional antenna is not a good solution for you because it has no ability to reject multipath, and all of your TV stations come from one location, anyway.

Your best solution is to mount a new antenna outdoors, using all-new equipment and cable, as I mentioned before. Your multipath issues will go away if your antenna is mounted at the highest point of your house, with an unobstructed view of the transmitters. Oddly enough, this problem is more often seen in very strong signal areas than in medium or weak signal areas.