2003 Nissan Micra Engine cutting out

When I am driving along, my engine sometimes completely cuts out but all the electrics and lights etc. Stay on. Sometimes I dont even realise it has cut out because it doesnt "shudder" like when you stall the engine and it doesnt make any noise. I only realise when I try to put my foot down on the accelerator and nothing happens, steering doesnt lock or anything. It starts up straight away when this happens. Also, when car is stationary i.E just started engine or stationary at traffic lights etc. The car will "shudder" as if I am about to stall the engine and I loose revs but have the handbrake on and car is in neutral so I am not sure why this is happening? Please can you help me