Maybe the dealer didn't do all their checks before you picked the car up. When I test drove one before I purchased it had just come off the delivery lorry the day before. It did have a rough idle, this was a manual transmission, and the dealer said it was because they had not carried out the checks on the car yet. It also had higher tyre pressure and thus a rougher ride. This is to stop the car moving about too much when on the back of the lorry.