97 Cavalier

I have a 97 cavalier that has been having a problem where when the fuel gets low, around 1/4 tank, sometimes a little more or less, it starts dying, typically when I'm idling or driving very slow, and especially when trying to go up a hill. When I fill the tank, it drives fine, and it'll usually start back up after it dies with a few tries. I have no idea what's going on, I had a new fuel pump put in in November but that didn't fix it. I just had it to a mechanic; he did some scan thing and said these boot things were cracked and replaced those but it didn't fix it. Someone told me it could be low fuel pressure b/c of a faulty fuel cap so I just replaced it and it's still doing it. I've also tried fuel injector cleaner. What's wrong with my car????