Mcrib

Hello, My 1991 I6 won't start and I checked the spark which it did have. Then I noticed that the fuel pump runs continuously when you turn the key on, instead of priming then shutting off. Also when you turn the key on the fuel pump relay switch buzzes, I put a different relay on and the truck started and ran but only for about 5 mins then it died and is doing the same thing again. I was wondering if you guys had any suggestions as to what it could be