are you saying when u rev and let go the revs hang a little before they drop - do u notice a slight sluggishness when u say rev the engine via engine bay/ throttle by hand - is there a slight misfire //delay when revving up? if so check the coil pack ecu wires ALSO power steering /connector / harness can cause same issue you describe - i left mine unplugged did exactly that - when the cu re-learns idle does cut out too - there are so many scenarios that can cause this - get it all diagnosed - or u can use forscan via laptop/PC pull up fault codes so u aint chasing ur own shadow..