2006 Chevy Cobalt dash board

I have an 06 cobalt and I recently was having a problem when I would go to the gas station and fill up when I would start my car it would start rough adn the RPM guage wouldnt read anything but the car starts. It has progressed now to along with doing that and the fuel guage will sometimes not read and go to empty while driving but then all of a sudden it will read the gas again. I recently found a fuel leak and it was repaired but these problems still exsist. We've replaced teh fuel pump in teh process of fixing the leak and sealed the fuel pump. I really need to find out what is wrong, Im a nanny and I don't want to be driving children around if this is a dangerous problem.