in my case it was a worn out housing bores that was not holding hydraulic pressuer on the pistons that keep it in OD. Mine started out like yours but I kept driving it as it got worse and worse.

Would fall out of OD into 4th as I rounded exit ramps and on ramps. Got worse and worse and finally would not hold 5th at all after the oil in the tranny warmed up.

There is a kit available that lets you replace the O-rings on the various pistons in the unit but that did not help mine. I finally replaced the whole OD unit with another from a junk yard.

Try the electrical route first, though. Make sure the solenoid is working. It directs the pressure to the right passages to hold it in OD.