When the engine warms up while driving the trans. will go to overdrive, but downshifts back and forth continuously at 50 to 65 mph. Kills gas milage. What controls the o/d?

 

It is automaticly controled by the torque converter. You need to flush and change your transmission fluid immediately. This fixed mine but mine was not that bad at the time. It only failed to hookup for the first 10 miles of warm up.
Now it goes into overdrive immediately.

 

You are feeling torque converter 'lockup' and this is pretty much normal at those speeds and driving with a light throttle applied. The reason this is more of an issue in older cars is because with the wear on the differential and inner CV joints, it is felt easier.
Now this is really going to "bake your noodle";;; when driving at those speeds and light steady throttle and you feel the shift and the RPM rises you have entered torque converter lock up and you are really running in a more fuel efficient mode. Then when you lightly step on the gas you'll come out of lockup and back into 4th, and the RPM will fall a little.
Consider torque converter lockup as running a hydraulic pump straight off the engine crank.