Problems staying started

Truck started cutting out when driving. It didn't actually ever quit running only cut out like it wasn't getting enough fuel. I figured my fuel pump was getting weak. One day it wouldn't stay started, so I thought the fuel pump went out and I replaced it with a new one and now truck will start but shuts off immediately. Wont stay running more than five seconds or so. I'm thinking maybe 86th wasn't my fuel pump now. But I haven't a clue what to check now. Please help.