I bought this van6 and a half years ago with 83000 miles on it.It now has 173000 miles on it.While driving on the AC Expressway I would fell a slight change in speed when I was around 58 to 62 mph. This happed only a few times.It felt ,for a second,like I was losing speed but it would go right back to normal.Could this be a transmission problem or an accelleration problem?