Volkswagen Sharan engine cuts out

Problem only occurs when the engine has been running for a while, at least 30 mins. If we stop the car for a short while, say not more than an hour and then start driving again the engine may cut off totaly, rev indicator goes to zero and may flicker up and down again. Power steering and brakes do not work. If we are travelling at speed the engine may start up again after a few seconds otherwise we have to stop. Engine will start fine and can normaly continue the journey with no further problems. For short journeys it does not cut off. I can drive for 4 hours, stop at a service for 30 mins and then when get going again it cuts out.