@opossum I did replace all my HDMI cables with premium certified cables for best results. I found that SNES still dropped sync intermittently only in 5x mode, though sometimes it only seemed to happen soon after having started a game, and then it looked like the signal stabilized. But I cannot say if drops would happen after more time because I did not test this mode extensively, since I decided that on this TV, for my taste, 3x looks much better (in non-PC, and any picture mode other than Game mode). These drops could also be due to something else in my setup, so it would be good if another c8 owner could verify.
I did not see drops in either 3x or 4x mode (though 4x I also did not test extensively because I did not like how it looks).
I do not have a Neo Geo so I did not test that system.