im working on a ef fairmont it behaves itself driving short distances bit when it goes on a long trip and you put the foot down to overtake etc it cuts out. checking out filter have changed the pump relay

Sounds like a vacuum leak
Could be split or loose vac hose, loose inlet manifold or even gasket is gone. Have a really good check use a bit of hose to listen or even use carby/throttlebody cleaner spray it around inlet manifold gasket if revs increase or change even like a miss or a cough you have found problem

If it all checks out ok and no vac leaks start looking at fuel pressure and even ignition timing

didnt think about air leak thanks

i had this problem with my EF i pulled over disconnected my battery to reset the ecu, reconnected and it was fine until it did it again a month later, then my car stopped so i changed my fuel pump relay hasnt played up again