1998 Chevy Astro fuel problem

Engine Mechanical problem
1998 Chevy Astro Four Wheel Drive Automatic 86000 miles

i am in ksa
i have astro 98' my problem I chainge fuel pump
3 times
now I put mercedes pumb. My frends tell me better
but he work 3month ok
now he working 3 hours then of
after 1 houer he is working again what can we do
thank u veru much