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Have you tried moving and/or turning the antenna? Since the antenna is in the attic, there may be some metal objects close enough to the antenna to affect its behavior.
Unfortunately, I think the main issue is that the antenna is in the attic instead of on the roof. The Noise Margins you originally posted were in the range of 4-12 dB. These are normally a bit too weak to be handled with an attic antenna. A pre-amp will make you see more signal strength (to deal with cable loss, splitters, etc.), but it cannot fix the inherent shortcomings of the antenna itself. Boosting a weak "muddy" signal gives you a strong "muddy" signal, which isn't really much help. Installing the antenna outside ought to clean up the signal.
The hope was that you might get lucky and have an "easy" attic install that behaves almost like a rooftop install. Based on your testing, it looks like that might not be the case.
If you still feel like doing some experimentation, it would be interesting to see if the signal dropouts/glitches are periodic or completely random. If you use a stopwatch to measure the time between glitches (either audio or video), do you see any pattern? Is the pattern the same on all the channels you receive? Is the pattern the same during the day or night?
Under normal calm weather, one would expect your signals to be relatively stable, with only very slow variations (caused by slowly changing atmospheric conditions). If your best channels (normally showing a quality of 80-90%) drop out every few minutes (quality suddenly dropping below 50%), then it would indicate something abnormal is happening (radar from airport? planes/cars/trucks going by? other local broadcasts? power tools/arc welding, or other local interference?).
If there's any way to figure out the cause of the signal disruptions, we at least have a shot at knowing what kinds of work-arounds to try.
Sorry to say this, but your best overall solution may be to give-in and move the antenna to the roof. No matter what is causing the signal dropouts, a rooftop antenna will add several dB of signal margin, and this will give your receiver more to work with.
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