First I would remove clean and lube (No rhyme intended ) the ISCV as there common for jamming.

If no luck I would check the CTS (Coolant Temperature Sensor) this as the name suggests provides info on coolant temperature to the ECU, from this info one of the things it will do is is open the ISCV (If required) and so the engine runs faster under cold conditions (I.E when starting).

The ISCV sounds as if it's working otherwise you would have no base idle at all, but you can always test when removed by putting 12V directly to it.

Martin