Car starts then cuts out

Hi i have a subaru impreza 2 litre sport, 1997, mailage 150k
drove the car 200 miles on the way home from picking it up from buying it. then the next morning i came to it and it turned over for abou 10 second and cut out. i took it to the garage to get a diagnostics on it and that showed a immobiliser fault. they said if i replaced the ecu immobiliser and key reader and key it would start. so thats what i did know the car is still doing the same starting for about 10 seconds and cutting out. the petrol pump and relay are both ok. so is the heater temperature swtch as that as also been replaced.
im desperate for help as i have spent alot of money on this car and no one seems to know what is wrong.