It's certainly worth a clean, but I would also check the co when hot, as a lean mix will also cause this, it could be caused by an air leak but would have expected this all the time.
When up to temp and no load electrical on(other than engine running) there should be no difference disconnecting the iscv, as it can only raise the rpm not lower it beyond the base setting which is too low by the sounds of it, around 950 with valve disconnected is about right, when the valve is replugged back in it will raise for a few seconds then settle.
tabetha