The command dials on many of these (now getting older) F5's get dirty or "plastic mould" forms on the wipers and circuit board strips making contact intermittent or unpredictable. The only way I have found to revive a finicky command dial is to use a shot of electronics cleaner through the slot of the offending dial and move the dial back and forth many times. Short of complete disassembly of the dial turret to msnally clean the boards and wipers, I don't know of any other tricks for getting reliable operation back.
