2009 pathfinder, when coming to a stop, engine revs up, then down suddenly.

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. Check the coolant level if it is low the computer can't read a stable engine temperature and the idle fluctuates. Could be the idle control motor vacuum leak or many other problems. Is the Service Engine soon/Check Engine light on? If so the fault stored in the memory of the computer of your car will indicate where the problem is.