Simaudio 600i V2 volume control.

@davt Hehehe! Glad it is working now - maybe @oddiofyl can explain what is happening. I am big Simaudio fan and love my 600iv2. So your unit was new in the box?

No, it was a used demo, probably 2-4 years old and has likely sat idle for a number of months. I am thinking that this idleness might affect the rather complex selection, volume dial. People have told me that the Simaudio equipment needs some time to warm up.        Or, Simaudio is made with spirits of long dead musical wraiths in the design, you know, like vacuum tubes, only not as unpredictable, and that every once in a while they act up. 

Ahh ok - I hope you enjoy it as much as I have enjoyed my 600iv2 - it’s my flagship amp. Yes, who knows what ghosts live in our machines :)

When a mechanical encoder is built there is a dielectric grease used .  That grease can get crusty and make it act exactly like OP is experiencing.   

I had a preamp and receiver that did that , I carefully opened the crimped body of the encoder , cleaned it, put it back together and problem solved.   It could be an optical encoder that's bad but I think it mechanical based on the symptoms.  

Thank you @oddiofyl!