Replacing selector knob

Might just be dirty pots. Try some Deoxit.

Tuner spray.

Is the "knob" bad or the selector switch? Replacing the switch would be a lot of work, so try a good cleaner/lubricant that has been suggested.

terminology: its the switch contacts that are flaky.  The know is the handle and is decorative.

The fix is likely electronics parts cleaner followed by a lubricant. but we do not know, you need to take it apart and also look for mechanical damage.

Its why i only design now with fully sealed relays under computer control.