Update -- did a much longer test drive (3x further than what it took the last two incidents to happen) today. No problem still.

One other think I have noticed is that even though I have my car on a battery charger, it is only 12.4V. When I have the car running, voltage goes up to 14V even during idle. So it looks the battery is weak. I have read weak battery could cause similar transmission error. I am going to have the battery tested next also.