Hi, i seem to be having issues with my set top box. Every once in awhile the picture will freeze and pixilate for a second. Also, some other times i'll lose audio completely.
Check all of your coax connections by removing, inspecting them for a bent center connector, and then reseat them firmly. Make sure you check all betweek the ONT and the STB including any splitters. I had the same issue and found a loose connector.
As far as i can tell its just one tv in the second bedroom in my apartment. I haven't noticed it on the other tv's. So it may just be what you suggested with the coax cable. It doesn't appear to be HDMI cable related as i tried two different cables.