1997 LeSabre has a miss

I need your help trying to track down the source of my problem.
When my LeSabre shifts into overdrive at about 50mph, it starts missing. Now, if I downshift it manually into the next lower gear to bring up the RPM's it runs fine. Or if I gas it and get the RPM's up it's ok. Another thing I have noticed is that it does not do this when I first start driving in the morning when the engine is not up to temp yet.

Now the short answer is to never put the car in overdrive until I'm on the highway running 65mph, but that still doesn't make the car right.

Does anyone have any suggestions on what to start looking for? It's had a tuneup, the tranny has been checked, my check engine light does not come on, and there are no codes on my code reader coming up. I am at a loss.

Thanks for any ideas you can give me.

Gary
Pataskala, OH