It could be crap in the injectors, but it could also quite possibly be a cracked or broken wire on a connector somewhere, the crank sensor for example. The tiny break in the wire may be making contact untill you brake for a junction or roundabout, the weight transfer of the engine or wiring moving very slightly forward could be enough to cause a momentary break causing the engine to cut out, when you come to a stop the wiring returns to its position and the connection is remade.
This could also explain why the fault doesnt show up on diagnostics, for the simple reason that it isnt there when the car is sat still idling in a workshop.
I think you should run the engine then have a really good pull and poke around the wiring loom in the engine bay, especially near sensor plugs, obviously be carefull what you touch around the ignition side of things.
No guarantees but I have definitely seen this type of fault on a car before.
Steve