While driving the vehicle started losing power, shut off, and would not start?

I was going thirty miles down the road and I noticed the car started having problems accelerating. It was acting as if our car was out of gas, but we had half of a tank. I put my foot further down on the pedal and it seemed to move a little more but was only going like ten MPH and it shut off. Once we tried to restart the car it would not start. So, we had it towed to our house the next day. Well, it finally started but shut off five seconds later. We were told it was the fuel pump, so we bought one and had it installed, and it still did not start properly. So, we took it back out and returned it to the store and we bought a fuel regulator, new spark plugs, and an intake gasket. I had them all put in and the car still is not starting. What could possibly be wrong?