Why does my N Reg Nissan Micra keep dying?

I will be driving along and the car will start to judder and then lose power; I have to take it out of gear and then put it in gear again to get keep moving! I'm getting worried becuase it's happened on busy roads and I could cause an accident... Also, it sometimes cuts out completely while im stationary at traffic lights.

Any help, or suggestions would be much appreciated!