it sounds like an electrical thing. where there is mixed signals going to the engine so it isnt working right at certain rpms or speeds. i would bring it to the dealer and get them to make up a work order to a certain amount. and then see what they say afterwards. it is a good way to get a quote and you can change your mind if it is more then what you agreed too. they will call and say what is going on. or that is what they do here.