>When I built them, they were solid, meaning that if I
> didn't move the slider, the value stayed consistent.

Carbon pots do tend to get crackly over time, and slide pots seem
to be worse than rotary, but that could depend on original quality

They pick up dust and carbon powder which can be cleaned out
by a lube like WD40 or even a water or solvent (meths) rinse

It's possible that the wiper is sitting on top of a piece of dust and
small vibrations are causing the ADC jitter

A capacitor at the ADC input should help integrate the jitter although
the RC effect will delay the correct reading when the pot is intentionally
moved.

Joe


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