check the wires to the alternator.
had this prob on the old fast ford RST. was driving back from jackos after just having a full respray and the cunt died on the M6

luckily i was following neil as the headlights on his car worked lol

got the services and checked battery termnilas, all was ok, so bought some jump leads and it started.

it lasted all to the end of the slip road and died again

turned out the cable had broken off the alternator!