Ford Escort TD p 1996 - keeps cutting out?

Hi all, my newly aquired ford escort td (60,000 miles on the clock) keeps cutting out, it doesnt seem to be any pattern to it, so days it doesnt and some days it does, but when it does it usually at low revs when approaching a junction, and sometime it restarts whn i turn the key sometimes it doesnt, then i have to try again and again until it does tick over again.

Any ideas

thanks in advance