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You have mistaken the signal quality meter on your tuner for a signal STRENGTH meter: the two are quite different.
Signal strength is a measurement of raw signal, and a digital tuner can see quite a strong signal and still show low signal quality if the signal-to-noise ratio of the data stream is compromised by interference. The symptoms that you have described, where signals come and go is a classic sign of multipath interference, which occurs when direct signal and reflected signal arrive at the antenna at slightly different times.
In the analog days, multipath was easy to spot: it showed up as ghosting on your TV screen: the phenomenon of seeing multiple images on a TV screen. Multipath is usually an urban issue, where TV signals are very strong: strong enough to bounce off objects like buildings, trees and hills, however, it can occur where indoor or-attic-mounted antennas are used in suburban and even rural settings, where the signals can be refracted by airplanes flying overhead.
Attic installations are usually trouble in areas of strong signals because the TV signal can often be refracted by some of the building materials used in the house. Modern digital TV tuners have increased resistance to multipath interference, but they are not completely immune to it.
The best solution to the issue of digital signal dropout, and low signal quality is to use a highly directional antenna that is mounted as high as is possible, aimed precisely at the transmitting tower, and free of close-by obstructions such as trees, hills and taller buildings. Please remember that preamplifiers can restore signal loss incurred by splitting a TV signal to multiple TV sets, or can help overcome line losses incurred by very long cable lengths, but are often mistakenly prescribed in strong signal areas for issues that are more properly addressed by using a better, more directional antenna, and precisely aiming the antenna. Preamplifiers can be a liability in areas where signals are very strong, however, because they often overload and cause a worsening of the signal-to-noise ratio.
A rotator is a very useful device when it is necessary to point an antenna at different transmitters. In your situation, a rotator and the right antenna system can easily double the number of TV stations available to you, and can minimize the issues of low signal quality and multipath.
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