97 SL2 Automatic Trans, will not shift into overdrive

[lotsoffear]

I have a 97 SL2 with automatic transmission, 220,000 miles. Still getting great mileage (32-36mpg). I drive 500miles/week and this is my third SL. Lost first to accident, second my buddy still driving. I love this car, just can't bring myself to get rid of it.

Anyway, just recentally it has started to have a weird issue:
When cold (first 5-10 miles) it will not shift into overdrive. It shifts into all other gears fine (well, as well as you can expect a 220,000 mile car). After the first 5-10 miles, I have to pull over, come to a complete stop and the take off again before it will finally shift into overdrive.
I have also noted that when I crest over the top of a long hill, it will for no reason shift out of overdrive and not want to go back in.
This has also dramatically affected my gas mileage.

Fluid is not low, no check engine or errors in computer...
Any suggestions?
Thanks in advance.