[wiswind]

It is worth having a transmission diagnosis done.....which is reading the codes.
There are transmission codes that a generic OBDII code reader will not read.....and have to be read at a shop that has the more advanced reader.

If the O/D light has flashed......then it has stored a code.
It is possible that this happened...and you did not notice it....as it is not normal to be looking there as you drive.

A high failure item is the Transmission Range Sensor (TRS)....also called Neutral Safety switch.
And it has a variety of issues that it can cause when it fails.

HOWEVER, take note that my '96 3.8L WILL, very often, downshift momentarily as the A/C compressor engages.....then it shifts back up into 4th.
This much more often going uphill.....less often going down hill.

So.....see if this happens with the air controls in "VENT"......which is certain to have the A/C OFF......