I have posted here before about not having overdrive on my V6 Camry. 156k These were the symptoms.
Overdrive light does not blink. (yes it works)

Trans shifts fine 1st, 2nd, 3rd.
Fluid was flushed 50k ago, new filter (no signs of metal)
Fluid is still clean and full
MPG is 21 to 24.
Every now and then OD will work for several miles then stop

Several weeks ago i was in it and the RPM's revved up to 4k, it acted like it was slipping, i let off the gas it went back to 3k and it was fine.
The OD light started blinking (First time it has ever blinked) and the check engine light came on.
I got to work had the code checked and it was PO500-vehicle speed sensor A and PO 720 Output speed sensor circuit

I ordered the speed sensor from Advance Auto installed it at NIGHT and had the same problem. The car will hold 1st gear to 3500 RPM's shift throw the same codes then rev up in 3rd then settle out for a while.

The next morning I pushed the power button on the console and it dropped right into overdrive. It shifted perfect went into overdrive flawlessly. I did turn the power button off to see what would happen, and it threw the same code revved way up then settled back. I pushed the power button it went into OD and worked perfect.

That afternoon when i left work OD was gone again, no codes, no shifting funny, just 1st, 2nd, 3rd. Half way home it was starting to get dark so i turned the headlights on and guess what came on too, OVERDRIVE.

For the past week OD works with the power button on and the headlights on. Turn the headlights off and OD turns off. Lights on,OD on.
This only lasted for a week. I am back to a three speed now. With the power button on.

I just got done cleaning all the connections and ground cables i could find, it had no effect. Anyone have any idea what is going on with this car?

Thanks,
Keith