The next time you try to start it, turn the key to the "on" position and see if you can hear the fuel pump energize in the back. If not, then there is a fuel delivery problem. Either the pump is dead, or it's not getting current. A bad fuel pump relay will intermittantly prevent the pump from getting power. You can check this by removing the relay and jumping pins 7 and 8 (I think) and seeing if that energizes the fuel pump.