2004 MG ZR engine dies whilst driving

I took my car in to have the clutch replaced. when i got it back it suddenly died on me in traffic after a long hot day. it sat at a friend's house for a few days and then i picked it up and it worked fine. a few days later it died again. i took it to the garage and they said it was the fuel relay, which they replaced. it died again, this time on the highway. what happens is the car suddenly begins to shudder and then dies. i can wait a while and then it starts up for a few seconds and then dies again. if i keep my foot on the accelerator pedal and rev it high it will start up, if i dont, it wont start. i cannot leave it to idle because it will die. could it be the fuel safety cut out valve? Air flow sensor?