2007 120i wont start after a trip

[wonderwall_85]

My friend bought a 2007 120i. what happens is it starts and then when we stop it after a trip(small/long) It does not start. It cranks but does not start. wait for anything between 40mins to 2-3 hours and the car starts fine. It does drive like its about to die and the fuel is about to cut off, all the time.
We have shown it to 2 different mechanism and they both could not figure it out. one did try spray fuel into the air intake but it did not start. I was thinking it could be the fuel relay or the pump itself but injecting fuel into the air intake rules that out I think.
appreciate any help