The shifter on this car is mechanical for park through drive then electric switches from 4 to 1. I wonder if the switch got something spilled on it. ( Later cars with the move left and right to shift 4 to 1 had a different set up. )

Try to get a hold of a scan tool that used the 38 pin under hood connection and access the trans computer. ( A 38 pin to OBD 2 cable won't do the job as it is the same as plugging in under the dash. ) Look at selected gear position and see if it is changing without moving the shifter.