gas gauge trouble and more

I have a 97 g/c laredo TSI after we bought it we began having problems with the fuel gauge. Once you had a full tank of gass you were fine but once you were off full it the gauge would go from full to empty constantly then you were on empty till you filled up the tank. Even when you know you have a half tank and shut it off once you started it again it would say that it was empty. We have no idea what it going on. Also, when you are stopped at a light it will stall out and you can started back up but also now it's starting to feel like it's slipping gears. does anybody have any ideas? This is all so frustrating:banghead: .