Why is my car spitting out gas, and it wont let me pump more than $10 in gas?

My fiancee and I have a 2001 Impala. When we got it, we went to put gas in it, and after $10 in, it started to overflow. so we took it back, and we thought initially it was a blockage. it wasn't. but it only spits out gas after it is started. so someone told us it might be buildage of pressure, so he changed the gas cap, and the vent, (i don't know, i think the pressure vent in/near the gas tank). now it spits out gas occasionally, but only while we are driving. also, it will rev on its own, and go from sitting still (the rpm's) up to 2-3, and then die if we are sitting still, like at a light. if we are on the highway, it just lunges bad. that is when the gas starts coming out. we have been to so many mechanics, and no one has even heard of anything like this, especially since we also had the fuel pump changed as soon as this started happening and it did nothing. one mechanic said that maybe it will be a part that is not supposed to ever need changing, or very seldom needs changing, maybe that's why no one is familiar with this. but my fiancee and i both think there has to be someone out there that this has happened to, or knows someone, or fixed a car for someone that did this, you know, something. thanks.