I would suspect a bad injector or a bad cat. You didn't say how many miles are on the vehicle but it certainly should rev beyone 4k in neutral even though it may not be a good idea to do so. If you have already taken care of the ignition side of the system then the problem has to be in the fuel system. Therefore, check the injectors to see if they are working properly. If the computer points you to a certain cylinder, that is where I would start. Also, the egr valve on the 3.1 is not hard to clean so you may want to do it anyway. Don't forget to clean the passage as well.