Help and advice required please.

It has happened twice now, starts fine, drives fine, then the first time stopped at lights the car engine cuts out and the reverse light is on (even when car was in first when stopped) gear display is blank and SMT warning light is on. Restarts fine and drives as normal for the rest of the trip.  Another time driving it was fine and then going from 3rd to 4th it feels like it losses power (or doesn't kick into 4th) and SMT warning light comes on. Stop and restart and drives perfect. Has a new battery and GSA has been replaced as has transmission fluid.
Any ideas as to what is happening, sensors maybe?