2000 Elantra kicks out of overdrive when it doesn't need to

2000 Elantra wont stay in overdrive after driving for awhile. Suspected transmission overheating, so we added an aftermarket cooler. Problem still exists.
Anyone have any ideas it would be greatly appreciated. Engine code said something about rpm to speed ratio incorrect.

Please help!

2000 Elantra wont stay in overdrive after driving for awhile. Suspected transmission overheating, so we added an aftermarket cooler. Problem still exists.
Anyone have any ideas it would be greatly appreciated. Engine code said something about rpm to speed ratio incorrect.

Please help!
What exactly is the code. If it says something like, 4th gear, incorrect ratio, pretty much means you need a new transmission.

Thanks lowsonoma, It's my friends car and I couldn't remember what code it was until you reminded me. I'll pass the info along. Thanks alot. I searched alot of other topics before asking and some were similar but not exact. Any idea what would cause this problem with the trans?