vauxhall astra merit 1.4

hi , i got a astra merit 1.4 m reg and it keeps cutting out now and again. sometimes it runs all day then cuts out, some times it cuts out every 15mins,I try and start it again but the starter motor just keeps turning, but if i wait 2mins it then try it always starts again. I have changed plugs,dizzy,rotor,coil and it still does it. Any ideas?
some one said it cud b modulator underneath coil, any one bring any light on this matter, cheers
andy