1995 Ford Explorer/250K well maintained. It randomly throws itself out of Overdrive and will not allow you to put it back in. Has since it was new. No rhyme or reason as to weather, conditions, time of year. Diagnostics show nothing. Been to dealers, shops and backyards with no luck. I expect to drive it for many years to come so I had the transmission rebuilt in May. Resolution to problem is to disconnect battery for 15 minutes and it resets. Gives warning that its coming by shifting hard into low nearing a stop. Best guess I’ve gotten is it’s electrical.
This transmission uses a transmission control module that operates all the shifting, including OD and the lock-up torque converter function. Chances are, that module is getting squirrelly.
Do you have a button that allows you to turn off the overdrive? If you do that’s probably causing your problem. You are either turning off the OD by accident or the button/switch is bad.
Maybe an iffy speed sensor. The transmission on my Lincoln ('96 4R70W) randomly dropped out of OD a few times and no codes were set even when it dropped for good. Another speed sensor fixed it right up. It’s also a cheap and easy to do repair.
The loss of OD was accompanied by no speedometer operation and inoperative cruise control. Hope that helps.