2000 Cougar Automatic v6

[bustin_skullz]

2.5L V6 Automatic Transmission
Having some issues, the car shifts great when running around town for a bit.
Once I hit the highway or drive for a period of 45-60 mins, the transmission really slams into first gear after that, and first gear only.

So driving around town quick car runs smoothly, once it heats up and runs for a while first gear gets rough and slams in, once the car cools down it runs perfect again.

Any help or thoughts?

[bustin_skullz]

*UPDATE*
I've been watching to see how long it takes for this to happen and noticed now exactly what happens.
While driving I notice the OD OFF light blinks on and off a few times, then the transmission light turns on.
After this happens the car after will clunk into first gear for the rest of the time I'm driving around.
BUT as soon as I turn the car off and back on the transmission light is gone and it drives perfect again.

Could it be electrical, tcu, or a sensor?