Your fault sounds like an engine shut down whilst driving and to make matters worse its intermittent.From garage perspective the most difficult of faults to diagnose even for a main nissan dealer! A back street operater will have NO chance fixing this fault.
If you are fed up bringing it to garages at a price everytime & you can live with this fault Id not worry to much about it.
It can be a number of things, in my opinion its sounds like an electronic control unit fault thats shutting the engine down or when the engine comes off a cold start mode in the morning and goes into normal running temp, this could also be a likley culprit.
Next time take note of few things.....
where is the engine temperature gauge needle sitting? is it in the same position every time the engine shuts down? also take note of where you are.....Did you go over a bridge , was it a bumppy stretch of road, is there an ESB station close by. All these things can influence the control units/electrics fitted to your car.
If you really want this fault sorted and since youve had it in other garages insist a top diagnostic Nissan technician is put to work on your car.Call Nissan technical dept on the Naas Rd, ask them to recommend a suitable garage with a technician that has expert diagnostic skills. Dont settle for anything less! Once Nissan ireland give a recomendation you have them in the loop should the fault prove difficult to find even for theor top technician.
some food for thought hope this helps
Dave