Vauxhall Signum engine cuts out

the car will start but can cut out straight away or drive normal then cut out mins later or hours later. it sometimes has problems starting it just clicks.i have had another starter motor put on but the fault is still their.it all started when i had the oil cooler replaced , before then the car ran perfect. it is dangerous and nobody seems to know what the problem is. i have talked to few vaxhall owners and they say when they owned a vaxhall they had a simular problem. is there anyone who can help please.