The drop shows the iscv is working, but it should also drop to it's natural base idle, unless the muppets have fiddled with this also, it's a brass screw on the underside of the throttle body, this is the air bypass, that allows air into the engine around the closed throttle flap, check the throttle is actually closing fully, leave the iscv disconnected for now.
The thing also is something is telling the iscv to open that much, 6K is a hell of a lot, I can't see it coming to this much with even a -100C reading!!
tabetha