1991 Buick Century Stalls when shifting into drive

The car starts normally and works fine but after awhile it will stall. I can start it again but it will stall when I shift into drive. I can start it again but will stall every time when I try to shift into drive.

Left the car for a few hours and started it again. Now it works normally. Shift into drive and away I go. Although without much confidence.

Any suggestions as to the potential source of my next headache. I would like to be proactive.