I have a 97 grand prix se that my brother gave me not too long ago. The problem is that when i gas up it goes to full but after 100 miles or so it starts going up and down until it's about empty then it stays on E. I'm thinking that it might be the sending unit but i don't feel like spending $350 on a new fuel pump just for that!! Has anyone had this problem before? If you have what did you do to fix it? Thanks!!