2002 Dodge Ram Fuel Pump Voltage

I have a 2002 Dodge Ram 1500 pick-up truck. One day it just quit running. I could get it to run for about 10 minutes then it would die. Now I can not even get it to start. I have spark. I have found that at the fuel pump, when I first turn the key on, I get 12 volts to the fuel pump but then it drops right out. My question is, does the pump have a constant 12 volts to it, or is there something along the way that turns the pump on and off. I have checked the fuses and the relay and all that is good.