We have a 98 plymouth grand voyager (3.3L) that did the same thing last night. Wasn't shifting correctly. At 40-50 mph RPM was around 4-5k. After a while of sitting off it seemed to correct its self (temporary?). Check engine was still on but shifting somewhat normally. 1st and 2nd gears seemd to down shift kinda hard. After a while the Check Engine light also went off. It is at the shop right now I will post what they think it might be. Unfortantly because we were in BFE, I was not able to grab in codes.