I know the KA24E ECUs have an idle knob on them from the factory, the SR20 ECUs might too.. There is a little hole in the side of the ECU, with a slot end pot (variable resistor) that can be turned for idle control.
If it helps you any, the KA24DE that's been swapped in my 240 has a vac leak somewhere and it dies a shaky violent death too. I don't care though, I'm getting an SR swap, and I don't need to drive it at the moment anyway.. But anyway, I'd look at vacuum leaks.