Why does the transmission come out of overdrive while I'm driving?

What is your question?

There doesn't seem to be any specific speed or conditions to cause this. I'll be driving along and can tell from the sound that the o/d has turned off. Other than the sound of the engine there doesn't seem to be any noticeable difference in the way the car drives.It doesn't happen all the time, only occasionally. Is there a relay or sensor that could cause this? Or, maybe a short in the wiring to the o/d light?