Welcome to The Ranger Station!
The IAC controls idle (air). Many times, cleaning the IAC will fix it right up.
Another thing to look for is any vacuum leaks. The IAC can compensate for a small vacuum leak, but not a gross leak. Spray car/choke cleaner around your vacuum lines/fittings and see if the the idle changes. If so, you found a leak.
You can test the Fuel Pressure Regulator using a fuel pressure gage attached to the fuel rail. First test for engine off fuel pressure, after or during turning the key to the Run position (makes the fuel pump Prime for two seconds).
Next is to test the fuel pressure again while the engine is running.
Then another test, with the engine running pull the vacuum line off of the FPR and note a change in idle speed and a pressure rise/drop. Continue this test by reconnecting the FPR vacuum line, note any change in fuel pressure, rise/drop.
Lastly, with the engine running and the gage connected, note the steady fuel pressure reading. Then goose the gas pretty hard, raising the rpms dramatically. You should see a change in the fuel pressure during the goosing and a return to steady when back at idle.
I'd tell you when the pressure rises and drops during these FPR tests, but I always get it exactly backward. You can also search on You-Tube for videos of the FPR test. Some are good, some are not so good, but it WILL show you how to connect the gage, etc.
If turning on the headlamps CAUSES a fluctuating idle, you should clean up all your major grounds, Battery- to frame to block, WS Wiper motor bolt to Head, and the small ones on the front fenders and radiator support.