Very often contacts have a thing corrosion-resistant plating. If you have got in the habit of abrasive cleaning every time you suspect a poor contact, you may be the cause of your own epidemic…
My bother surprised me recently by showing me the can of Servisol he used on the valves in his Marshall head (it had briefly cut out twice). I hadn't seen a can for decades. He demonstrated how another amp (a small Vox) had a faint tinkling/ringing harmonic to its sound, a problem since new a couple of years ago. I gained brownie points when I not only diagnosed valve microphony, but identified the dodgy valve by pinging it with my fingernail (when turned off, of course). A swap out cured it!
So my suggestion is find some Servisol.
Neil