it has to be an ISV problem as this is what keeps the revs at a constant 1k rpm.
when the car is cold the ISV will open the throttle that little bit extra to keep the revs that bit higher
when driving the car and coming of the throttle, say changing up and down gear do the revs plummet or do they drop slowly
if they plummet the ISV aint working, they should drop slowly
also, check and feel if the ISV is gently vibrating when the engine is running.
or easier would be to disconnect the ISV, start the engine - then feel the ISV
stop the engine, reconnect it, then restart and feel the ISV - you should feel a difference
my dodgy idle was simply a dirty terminal/switch on the ISV temp sender