Nissan X-Trail: Dealership isn't fixing my car

A young friend has a problem with a Nissan X-Trail where the motor will either cut out at suburban traffic speed, or run very rough and lose all power, which is extremely dangerous. Unfortunately it appears that it is beyond the Nissan dealer's ability to fix it. Nissan dealers have had the vehicle on their diagnostic rigs and said there are no electronic faults being registered. We have done all of the other things, having the fuel tank and lines cleaned, replaced fuel filters, spark plugs and leads. The way the engine behaves when it restarts suggests it is not a fuel related problem and it is now a case of possibly replacing the various sensors in the probability that one of them is doing things that are not registering a fault on the diagnostic rig. It is very disappointing that Nissan dealers cannot fix their own cars and car owners are expected to go through the expensive process of replacing bits until they eventually cure the fault themselves. It raises serious questions about commonsense and the use of diagnostic rigs to say there are no faults in an engine when there obviously are. This raises the question of whether the X-Trail should be subject to a safety recall when it could lead to a very serious accident. The young lady who owns the car is naturally at her wits end and any suggestions your can provide would be appreciated.