I would start by turning off any computers that are on the same circuit as the TV, or near the attic antenna. My computer speakers completely wipe out VHF 12 on my TV, even with a rooftop antenna.
The wild swings in signal strength usually indicate multipath signal reflections which are probably causing most of your reliability issues. Different mounting locations in the attic may help somewhat, but no guarantees. Adding any kind of signal amplification will not help the situation.
You will likely need to get something outside on the roof to improve reliability on VHF13. Do you think you could get away with a
small antenna on the roof?
Hopefully others will chime in with some suggestions.
