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The way those systems usually work is that there are On-Off valves, a Variable Force Solenoid valve and a Pulse Width Modulated valve in a solenoid pack in the valve body of the transmission. The On-Off valves schedule the shifts by a sequence of one valve on while the other(s) are off for one gear, The opposite valve on while the first is off for another gear, both valves off for another gear, and finally both valves on for another gear. There are engine temperature sensors, throttle angle sensors, engine speed sensors, transmission speed sensors, and in your case an axle speed sensor. When the proper engine speed, engine temperature, throttle angle, transmission output speeds are achieved for a period of time the Powertrain Control Module sets the status of the shift control solenoid valves to select a particular gear range. In your case you want over drive. So it appears that yours is capable of selecting the gear by a setting the on-off state of the shift control valves. However your wants to drop out every so often. That tells me it can be an intermittent sensor, a bad wire, or an intermittent connector from corrosion. It also could be a bad Powertrain control module or a chaffed wire somewhere in the loom. I gueessed the speed sensor because that is where most problems occur. My guess.
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