McIntosh MX119 Surround Mode or Volume knob issues

If it skips modes, it sounds like the internal momentary contact is not releasing as it should.

If your front knobs are activating electronic contacts on a circuit board, it is still possible to clean things: I have repaired many remote controls by disassembly, removal of rubber layers with buttons that make contact with contacts on circuit boards when pushed, some knobs that are sticky that stay down, ...

IOW, it's possible it's not the electronics, just the mechanics of the knobs, layers of rubber, printed circuit contacts.

Alcohol, brush ...

Looks like a nice unit, and you got it for a good price.

Sounds like a trip to McIntosh repair.

Just to rule it out: It worked well for a time that you did not return it to seller, correct?

I suggest you try/clarify things for yourself and to tell the tech.

Clean rear jacks and switch cables left to right, see if problem moves?

Does yours have McIntosh Mode Control: Stereo Reverse, L to L&R, etc.? You may get some clues or confirmation there.

My vintage equipment has controls that can be reached by removing the front panel. I hose the controls with contact cleaner/lube. Yours may all be electronic, I don't know anything about it.

Any possibility dust, tobacco smoke, cat hair, anything that could get in the front controls.