My 2000 renault megan 2ltr engine cutting out?

After driving the car for a bit sometimes the electrical warning light will come on and the engine cuts out and won't drive. Garage hasn't a clue why it does this. Will start ok, but will only drive after about an hour. concerned as it cut out while driving in mid flow traffic and stranded there. I've had a new crank shaft sensor fitted, and still it happens. Anyone go any ideas?