Clearly the ISCV is working and doing it's job, the problem is somethings telling it to open by the amount that is causing your fast idle. When you unplug ISCV you have a mechanical base idle so it can not drop below what it's set at.

I should think 750RPM is a bit low, what can happen is if the idle is to low ISCV will bring the revs up for compensation then when the revs drop again to base idle it brings then up and again and so the cycle goes on.

So it depends on what you mean when you have turned the screw, I.E have you tried decreasing or increasing?

If it's not then then you need to be looking at what info your ECU is seeing, I would first be looking at TPS calibration and if not this then the CTS.

Martin