ADC values from Pot 100k oscillating but fine with 10k Pot, why?

I’m trying to put controls onto my Vintage Pi Radio which lets me listen to UK radio over here on the continent.

I have 3 Potentiometers, 10k from a kit, a 100k one from Pimoroni and an old one in the radio which I think is about 2000k.

When I connect 5v to the pot and test the analogue input on the arduino, the 10k works as expected as does the 100k, but the 2000k increase and decreases as the pot is turned but the value is unstable and oscillates by about 25% in a sinuous amount.

If i connect all of these Pots to an mcp3008 on the Raspberry Pi on 3.3v the same effect is observed but also the 100k starts to oscillate by about 10%.

Can someone explain why this occurs with higher resistance Pots, and how I might use the original 1000k Pot if at all ?? It seems a shame to have to replace it.

many thanks if anyone knows the answer.

The older pot may just be dirty. You may have to spray the insides with contact cleaner etc. Also keep in mind some pots are linear and some are logarithmic. Volume controls are usually logarithmic.

I wonder if a capacitor across the pins of the potentiometer might work to smooth the output?