Subaru Repair: car starts then cuts out, mass air flow sensors, air flow sensors

Question
hi first can i say thanks for your time.
right heres the problem, i bought my subaru impreza 2 litre sport the other week, drove it home 200 hundred miles and it was running fine had to fill the tank up as it was on empty, (some people are saying it could be the fuel!). then i came to it the next morning the car turned over, stopped on for about 5 seconds and cut out as though some one ad turned it off. i kept doing this so i got it took to a garage for them to run a diagnostic on it and it came up with an immobiliser fault, the mechanic then told me to buy a new ecu, immobilser, key, and key reader and it will work. so i got the 500 pound together and done that, and the car is still doing the same. it still turns over stops on for about 5 seconds and dies. can you please help as no one seems to know what it is.ive spent alot of money on this car and i need a car to get around to take my kids to see there grandparents ect..
i would be greatfull for any kind of information you have
thank you nicola

Answer
Need more info.  Year, model type,  US built or Euro built?  Who did the repair?  Have seen bad BIU, but not ECU. You would have need to replaced the BIU, and all the keys, and had them all reprogrammed by the dealer. IF the ECU was replaced and not the BIU then they were probably wrong in diagnosing this as you are still having this problem. You should not be liable for the mistake. We have also seen bad mass air flow sensors and will do the same thing. I hope this helps.