how can i fix my 1998 ford fiesta fuel pump problem?

my 1998 ford fiesta would not start, I can turn the engine over but it would not fire up, I had the r-a-c- come out and after checking the relays / fuses he banged on the fuel tank a few times which got the pump working, got home and went to start the car but the same problem, could not hear the motor when I turned the key, so had the fuel pump changed. The car started for 3 times then no more, plus no sound from the pump when turned the key, could you help with some advice as I cant keep replacing the fuel pump