I recently purchased an SNES (again) and I've noticed that the video output to the television has some slight interferance that creates a wavy pattern visible over the actual video signal. The video displayed is not wavy, mind you, just a kind of overlayed sheet of waviness that is really visible in bright areas, and nearly invisible in dark ones.
I've tried two different (old) coax connections, and a new composite cable from my GCN, but they all have the same problem. I've tried different games, with or without a controller plugged in, on two different televisions in two different houses, through an RF signal booster, through an RF modulater and without and still it persists. My suspicion is that the machine itself is interfering with the video, or the power cord might be doing it; it's one of the few remaining things I haven't tried swapping out.
Frankly I'm inclined to just live with it as I hardly notice it after I start getting in to whatever I'm playing, but there's no reason not to try and fix it if I can.
tl:dr Have SNES, video is wonky, think it might be your mom, she is fat.
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Maybe check where the connector that goes into the TV goes into the SNES and see if it's wobbly, it might have a loose connection to the SNES board.
If not try using a SCART/AV cable from a GC and see if that helps.