My guess is that the coaster brake is causing it. To be sure I would touch the rear hub the next time it happens to see if it is getting hot. If so that means there is fricton building up in there creating heat which tends to cause the metal parts to expand just creating more friction and heat and so on. When you stop and let it cool down the parts contract thus reducing the friction and allowing the wheel to turn again. I suggest getting the coaster brake serviced to prevent this from happening.