extremely odd hum problem

I've been having a problem with the right side of my recapped BS 5500 lately (well the left side is acting up again too but that's another story).

There is a really funny hum on the speaker outputs. the speaker selector knob goes [B+C, A+C, A+B, A, B, C], so 6 different settings. with the speakers hooked up to B, the hum is loudest usually.

With the speakers hooked to A, sometimes there is no hum at all on A, but a high pitched hum on A+C and low pitched on A+B. or, any other possible random combination. After a couple minutes after power up, the hum will stop when switched to A+C, with A+B and A still humming. Or, A will stop and A+C and A+B will still hum. Or, they will all quit. Or, they will all keep humming! I've tried hooking the speakers up to different jacks (A, B, C) but the pitch of the hum is always a different random combination, and when the hum stops is always at a different time. But it's always there on power-up. I didn't have this problem until I sprayed contact cleaner on the selector. The cleaner got rid of the static between switching, but it seems to have caused a hum! and jiggling the knob doesn't do much good.

EchoWars (my expert tech) suggested that it might be a zener diode or transistor, but sometimes clicking the knob back and forth changes the pitch of the hum, so it might also be a selector problem.

I really wonder WTF is causing this. It's annoying the living crap out of me!