2000 Chevy Monte Carlo fuel

Hi,
A few weeks ago this car began showing a false readings onthe gas gage, thus resulted in me running out of gas. However, when I put gas in it - it doesnt hold very much because it actually has gas. Now the kicker is this- when I do put additional gas in, it cranks up and runs perfect. At first it stopped working at 1/4 of a tank and I would watch it. Now it has began doing it at 1/2 tank. Again, there is gas in the tank but it will start up and run when the new gas is put in the tank. I hope you can advise. Thanks!