My 5 year old Sony MHCMG110 mini stereo has a problem:

The volume knob control doesn't always work properly. Before I open it
up I thought I'd post here and get some guidance.

The volume level heard appears to correspond correctly with the digital
level displayed, which goes from 1 to 30, maybe higher. I never have it
higher than around 20. However, when spinning the volume knob those
numbers don't always advance or decline as expected. Sometimes they even
go backwards briefly!! When using the remote control, the volume changes
smoothly and properly.

My first thought was a slipping pulley, but how would that make the
volume go the wrong way? It doesn't go the wrong way a lot and not much
- generally one digit and only momentarily when spinning the dial. Maybe
it IS a spinning pulley. Anyway, this thing is pretty big and complex
what with all the features (and I'm therefore hesitant to open it):

60 CD changer
2 cassette bays, one recording auto-reverse
AM and FM

The speakers are separate (this is the stereo version, not 5.1 or 4.0)

I bought the service manual yesterday online (they emailed it to me) and
I printed it out. It's mostly full of the schematics, but there are
parts diagrams and 3D assembly/disassembly diagrams. Looking through the
whole thing I didn't spot anything that would directly relate to this
problem. I figure I can either live with it or open it up and try to
find the problem, maybe replacing a pulley or applying a rubber
rejuvenator (recommendations?). Or, I could get a learning remote and
use IT to change the volume (the one that came with the Sony has a kind
of weak signal). I'd rather fix the spinning control than have to use a
remote. Thanks for any guidance/help.

Dan