After replacing fuel pump assembly kit and filter fuel pressure not holding correct psi

After replacing fuel pump assembly kit and inline fuel filter the truck has a normal psi when they key is turned on, but when you crank it the psi drops to 40 to 42 psi and does not run properly. Any ideas what else might need fixing or replacing? I have only owned this vehicle about three months when it messed up on me. From the day I bought it I have kept 3/4 to a full tank of gas in it to avoid this problem so I do not know what I have done wrong, lol. Please help!