I have an LX 5.0 automatic and when driving on the express way I normally engage the overdrive around 55mph. Now when I step on the gas my RPMs jump up but the car just don't seem to go. When I let off the gas the RPMs normal out. I have rebuilt the transmission and changed out the transmission belt but it still does not work. I live in Korea via the US military and Koreans have little experience about foreign cars. Can anyone recommend a fix action.