1.8lt Idle dipping to near stall, fluctuation when lights are on

On my 1993 Cabriolet I've been trying to find a gremlin that causes the idle to rise and dip (to near stalling levels) when power is being drawn (in this case, when the headloghts are on - theres no air con).

At first I thought this would be the Idle Stabiliser Solenoid/Valve and Shughy kindly sold me a replacement, however the problem persists. The solenoid is getting power as I can feel it clicking when I rest my fingers against it and turn the lights on.

There appears to be no leaks in the vacuum hoses.

It's almost like the stabiliser solenoid increases the idle too much and so clicks off, only for the idle to drop too low and it to click on again, and so on and so on.

Would anyone know what else I should be looking for here?

Many thanks in advance

Get the car nice and warm and try increasing your idle speed (screw on the back of the throttle body) to about 1200 rpm then lower it to 1000 rpm.
I recon your idle screw has become lose and the car is running on the ISV and when you add the extra load of the lights is making the ISV kick in and out. Same thing happened to me and the revs went up and down, I increased the idle speed and mine was OK after that.

edit… Not sure if you have the k-jet fuel system as thats the system I'm talking about.

try unplugging the valve and see how you go, the early mk1 didnt even have this setup