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Well using another TV SNES #2 and #3 put up a light to moderate fight but got the right result. That removes the a/v cable of your choosing and the TV (jacks and all) being the criminal in it too.

My next step would be a fresh OEM power supply, it still could be a voltage issue. I'm recalling more of that post I talked about, and the dude was using the stock system parts and all but it was also in relation to not just real games, but kits and very needy kits too like the SD2SNES which draws the most power. That's where that 1 loss of volt on a 3rd party vs OEM AC adapter screwed it up to malfunction.

You asked about the AC going bad, they can, they do have a fuse inside that could be weakened but not busted, same with caps too (same can be said of a fuse inside the SNES itself.) It could be going bad, but not quite dead yet. The other and it's already said, nastied up cartridge slot may cause it too but it would be strange as they either just work or not, not get crazy like that but more like with garbled visuals, not a total drop out.

If you have a SNES cleaning kit from Nintendo, instead of water, soak the console insert cartridge held pad with 91% isopropyl alcohol and with it unplugged give it at least 10 pulls, perhaps even 10 holding it kind of back, then 10 forward (like if you were rocking it, but not.) just so you scrape all the pins equally as there could just be funk. It's a good way to eliminate a pin issue, so could using a safety pin in case it's just a looser fit than normal.

Yeah, absolutely. If you're using a flash cart, they pull more power.

Interesting, thanks for that. I think I'll pick up another OEM power cable some time to try and eliminate that.

I don't have a cleaning kit unfortunately (doesn't look like one was released in PAL territories - or there just aren't any on Ebay). I've heard a piece of card (e.g. a credit card) in soft cloth with Isopropyl alcohol can work. Is that worth trying?

Yeah, that's definitely worth trying if it keeps giving you problems. I'm sure there's all sorts of tutorials on the best materials to use, but something credit card-sized should work just fine. Just make sure it all dries out well before using! (One of the reasons you want to use 91% isopropyl alcohol, it will evaporate quickly.)

The best replacement would be a very thin shammy or one of those smudge free silk like glasses wipes. Tightly wrap around a thicker card like credit or two to hit the thickness of the NES board or so, soak it the same with the same stuff, and give it the same in and out treatment and it works all the same.