Idle surges with electronic load: Villager/Quest

I have a 93 Mercury Villager with 200K miles on it. The idle surges from about 800 rpm to 1500 rpm whenever something electrical is turned on, like the headlights, engine fan, or even a power window. I tried adjusting the IAC valve all the way down but this surging still happens. Recently we had the intake manifold tightened and there are now no vacuum leaks. A year ago we had the cylinder head and manifold gasket replaced but this problem cropped up maybe 6 months after that job. I could have it diagnosed at Nissan for $140 but I want to see if anyone has had a similar experience first?

Monitor the battery voltage when one of these components is turned on perhaps a clue is generated.

The IAC can be adjusted? That is the first I have heard of that.

When you turn motors on, it’s an electrical load. Your vehicle may have an electrical load detector which sends a message to the engine computer. The engine computer may, in turn, command increased rpm to produce more electrical power from the alternator. These are all “may bes”. You need the repair manual for more definitive answers; or, the dealer’s services.
You could have the battery and alternator evaluated. The charging capacity of the alternator may be down, or, the battery may not be charging well.

And there are no stored computer error codes?