i have a 2000 dodge neon, automatic transmission, and recently it has been lurching forward on the highway at certain speeds. i have been keeping my foot steady on the pedal but at like 75 mph the car will lurch forward and backward and the rpms will jump between like 3000 and 4000 (the car was steady at about 3000 to begin with). when i go down to about 70 the car drives fine, so what could be the problem and how can i fix it?