Or it may be that the one with the bouncy needle had a problem in the speed dial resistor or one of the meter cells. The CdS cells in the meter, as semiconductor devices, could potentially fail in a manner that would make their conductance voltage dependent, like a zener diode (though more commonly they fail as simple open circuit or high conductance, insensitive to light). A noisy (i.e. dirty) resistance strip or wiper in the speed dial could do the same thing, with increasing voltage amplifying an effect that's still there, but less noticeable, at lower voltages.