My car will start on the odd occassion but cuts.

My car will start on the odd occassion but cuts out after a short time, have changed the relays both the fuel and shut off, when I disconnect the fuel line to the injectors and the key is turned there is fuel but then the relays click and no fuel, could it be the filter or pump although the pump appears to work ok when driving or when the engine runs sincereley brian jackson ps I live in thailand so very difficult to get an honest answer from the so called mechanics, they just say fuel pump then start adding things on