i have a 6.5 diesel vin f engine it keeps shutting down i have replaced the pmd on 1998 Chevrolet Suburban 2500

i have replaced the pmd 3 times still shuts down could it be the lift pump? i also know it could be the oil sending sending unit.

Asked by for the 1998 Chevrolet Suburban 2500
do fuel pressure test cold and while driving
what should the fuel pressure be? could this be a bad lift pump? thanks for getting back to me i have been fighting this for weeks.
approx 5 lbs all the time
ok thanks