[morzh]

Use (don flood!) good grade penetrating contact cleaner (many of us use Deoxit D5), spray some inside the switch ()they likely have some sort of openings for it to penetrate, and then work the switch several times. I was able to do that to a close volume control switch yesterday. Do this for every switch.
Then check them for functionality.
Then see if the problem goes away; if not, see if any rewiring was done; if not, start understanding the electrical aspect of it.
Kinda what you did to the 666.