Yep - What Tom said.... Also, my GRC-9 does the same thing occasionally. Found to be tin whiskers growing in the apparent tin plating of the cap plates. Business card or pipe cleaners first, then a blast with compressed air removes them. A few microns in diameter, hard to see, but the ohmmeter can't be fooled... In my set they shorted out the local oscillator cap at some rotational positions..
Some real physics "bugga bugga" going on there. A known problem with tin in electronics....
Give it a try..
Tim
N6CC