Hello, I have a 1994 Ram 2500 Cummins diesel engine. Great vehicle with minor problems. My questions is while driving on the highway on a flat surface with cruise control on or off at a constant speed the vehicle increases RPM by 300 to 500 and then settles back down. So 74 mph RPM = 2000 every couple of minutes it will increase to 2300 for about 15 seconds then drop back to 2000. Dealer says they can not find a problem. Well this happens every time I drive on the highway w/o a load so they are hoping I go away. Does anyone have any thoughts on what could be wrong?