I had the very same issues with my OD, even intermittently turning off at bumps on the road, including at high speeds of 75-80mph (annoying!!!), delayed use in lower temps when I had to "wait" for miles and miles until I was able to turn it on...just to learn that it would keep popping out soon after... lots of holding the shifter to the right side while driving... stupid and dangerous. Later this would worsen too, holding shifter to the right wasnt enough...it happened more and more often after hitting less and less bumps or regular road vibration or even a slight tilt in the roadway was enough to trigger the unwanted cut-out.
I thought the OD internals were failing but now I know all is fine.
I think your 4th gear switch in transmission is worn out.
If you decide to go under the car, disconnect the 4th gear contact switch on the top of the tranny by removing the single wire that runs to it. Not the one with two wires, that is the Reverse light switch. Find the single- wired one on the right front side of the top of the tranny. Google some pictures before it.
Just unplug it carefully... Try to not break the plastic. Ground this wire to the negative on the tranny or to the negative tab on the solenoid. Dont mix the polarity
it must be a negative.
If you do this mod, you will be able to turn it on EVERY time in ANY GEAR. yes even in reverse so be careful DONT ruin the tranny. IDK if using od in gears other than 4th gear can hurt it or not.
Avoid forgetting OD on, use it ONLY when intended: in 4th gear!! Avoid accidentally using it in Reverse gear.
If your OD will work fine with this mod, you will know what to order: 4th gear contact switch, also called an overdrive interruptor switch; pictured above earlier.
My OD never popped out of 5th gear ever since, not even once. And it's been years, operating fine. I should finally buy the damn switch and turn it into the tranny. An easy project, no lowering of tranny is needed but it is a hard to reach spot.