The fact that it made any difference shows you something was faulty.
Just wait till it gets fully warmed up, disconnect the ISCV, then will all electrical loads of apart from the engine running adjust to around 900 rpm, reconnect the ISCV, it will probably raise a little bit to start with, then settle to the base idle you just set to 900rpm.
The engine can only attain such speed that is has air for, so has to be getting in somewhere, most likely scenario is that it had a dodgy cts previously so people been fiddling about with base idle screw, on underside of the throttle housing to bodge it, but of course in the process not rectifying the actual problem.
tabetha