I had a Mercury Sable doing the exact same thing. It took replacing four engine sensors to fix it. But I think they screwed me on the first three.
Some kind of liquid was dropping onto the sensor and making the RPMs go down when in idle.
Did you notice the appropriate size of the booze bottle he said not to use for cleaning in the end. That size comes in handy if the problem
doesn't get fixed. But the bottle was empty.