96 stratus intermittent downshift

2.5 L v6, occurs when engine/trans cold, within 1-2 miles after driving, at speed 35 - 45 mph. Trans will downshift to a lower gear. Have pulled over, and stopped, trans still in drive, and begin driving again. Trans works fine. So far it has occured 4x past 2 months (Dec & Jan). The speedo was displaying correct speed. Shifter in "D".

Is this a trans internal malfuction, or some external sensor, computer mod, or? Any other things to look/listen for the next time this happens; to help trouble shoot.