Idle wont adjust on electrical load ('93, JH 1.8 injected)

I've been battling this problem since buying the car about 5 years ago.

Issue is the engine won't maintain idle when the headlights are on. It will dip to 500rpm, the IAC valve will kick in and boost it up to 1500rpm, then it will dip back down again. This repeats rythmically. It's like the idle doesn't adjust when there's load on the electrics.

An easy fix is to manually adjust the idle screw to increase the idle RPM, but that means you have to turn it back down again during the day when the lights are not being used.

In fixing up the car I have replaced:

 - IAC valve
- Battery
 - Alternator and belt
 - Fuel filter
 - Fuel pump
 - Lifter pump

So I don't think its a fuel delivery issue, or an electricity generation issue. I've also checked for vacuum leaks. Car drives fine, otherwise.

Would anyone know of what I should be looking for?

What is your idle speed at the moment?

I've found a 1000rpm is where my car is happy and if I turn on lots of electrical stuff it only drops to just below that.

Have you checked the rubber O-ring on the idle screw as they do go hard and flat which causes the idle screw to move on it's own.

I get the engine nice and warm and adjust the idle on the idle screw up to 1200 rpm, give it a couple of blips on the throttle and check it stay at that speed then lower the idle speed down to 1000 rpm then blip the throttle again and check it stays at that speed then try the lights etc….

This might be different for your car as I think you have digifant fuel injection and I've got K-jet?

Hmm haven't checked the O ring, but maybe the screw is moving - I often adjust it.

Worth checking out, thanks1