1998 Buick Century rough running engine

I have a 1998 Buick Century with 3.1l V6 engine. When driving in the highest gear and need to accelerate - the engine runs rough. When then downshifted, the engine runs fine. It runs fine when acellerating from a stop or in any gear other than the top gear. It runs rough under even a mild acceleration - not full throttle - in top gear. It runs just a little rough at idle. There are no engine codes.