Flaky Vortex knobs

[Jeff Larson <Jeffrey.Larson@Sun.COM>]

I just bought a Vortex from eBay, and it appears to have a serious 
problem with the Value
knob used to change parameter values.  It sometimes decrements when it 
should increment,
ignores a click, overshoots, or rapidly bounces between two or three  
values as it turns.  I can
eventually dial in the desired value but it takes forever.   Likewise 
the Parameter knob seems
to sometimes miss a click so you end up editing the wrong parameter.  
The Preset/Register knob is fine.

Does anyone have experience cleaning and/or replacing these knobs?

Thanks,
Jeff