Maybe try some electronic contact cleaner? Be careful with it, though. It should be ok, but I don't know what it might do to the touchscreen.

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I think it might be a chip or software problem. I doubt the knob is connected to a rheostat.

 

@donkpow - Not a rheostat, but it is still a multi-position switch. Whether it uses electrical or optical contacts, it may just be some dirt in there causing the problem. I suppose a blast of compressed air may work if it's optical.

 

The volume control is usually a rotary encoder:
https://electronics.stackexchange.c...ions/443918/how-to-best-clean-this-sealed-mechanical-rotary-encoder-volume-knob

It is probably the cause of the problem, but there is a chance there could be a loose connection or cracked solder joint near it. Cleaning it from the outside may not work, but is probably worth trying. You should mask off the area around it with tape, because as OutbackJon said, contact cleaner should be kept off the display.