Hi there.
I have this EXACT problem with my ls400
Everything works as it should cold start, increased idle etc.
But as original poster describes, the revs will not drop unless the car is bought to a complete stop.
It's as if the cold start system is linked to a speed sensor or something that prevents the revs dropping to normal once engine is warm unless you come to a stop.
It's obviously a Common(ish) fault as the original poster had described my symptoms to the letter.
Someone must have come across this and any help would be great.
Many thanks
Gary.