When you say that you tried contact cleaner, did you just remove the knobs and spray in the opening, or did you take apart the system and actually spray inside the switches? If it is the switch contacts, jiggling the switches should have a similar result with the crackling. Sometimes switches need to be disassembled and cleaned manually or inside an ultrasonic cleaner, if that's the cause. Cold solder joints can also lead to intermittent problems like that, but that requires taking the system apart and doing some proper diagnostics and repair. How difficult that is depends on the internal design.