Fix Fiesta 1.25, keeps cutting out?

i have a 1998 ford fiesta 1.25 zetec. recently it cut out and would turn over but not start. ive changed the fuel filter and fuel pump, then the coil pack. it worked for a bit then cut out again and now it wont start.
if it does start eventually it cuts out after about 30 seconds or so.
please does anybody have any ideas about what it could be? ive only just bought the car a few months ago as well! thanks