After changing my fuel pump, my van start giving me problems

First of all sorry if my english is not that great. Hope you understand!

After my fuel pump got changed, everything was great(so I thought). After I changed the fuel pump, my gas reader would always show full, so I had no idea of knowing when I needed gas. This problem, I found later after noticing the gas wasn't going down. Well my trip was the next day, so I kept track of the miles and went on my 4 hours drive to TampA(my van had always take me there with no problem). After 3 hours and 55 minutes drive(5 min from my destination), I got to a red light and my car rmp dropped and van turned off(yes, I put gas on my car). It turned on right away but turned off again. When the light turn green I drove to a parking lot very slowly and rmp dropped again. I needed to be somewhere, so I could not wait. I called a tow truck and left. Later on, they called me telling me the van is running fine. I could not believe it. The mechanic drove the van for an hour and no problem. They also did a diagnostic and no problem. They could not find anything wrong. They removed the fuel pump and changed the fuel filter. Eventhough, we paid to have the problem fixed the van gas reader never drop. Again no way of knowing when I was running out of gas. On my way back, my car started hesitating again, so we turned off the van and waited for one hour and that did the trick. She was driving like a champ. I am planning to take the car back to Good Year since I have a warranty for a year. But I would like to know. What's wrong with my van? She never ever gave me problem until the fuel pump got change. Could it be water on the fuel tank? Thank you!