2007 Audi A6 revving problem

Sometimes from when i turn the car on till turn off , when releasing the throttle the revs dont drop to 1000rpm like they normally do, they stay at around 1500 bouncing from 1400-1600 rpm until at a dead stop then they return to normal, but like i said it will happen on the way to work but be fine on way home or visa/versa
Any help will be great
Thanks
Ryan