Why does the fuel gauge needle oscillate wildly when the gas tank is under half full on 1997 Buick park avenue ultra?

If the guage is acurate in the top half of the tank I would assume that the sensor mechanism is ok and that you have an electrical fault. I am not familiar with 97 buicks so perhaps look for a better answer. The lower half of many fuel gauges is problematic so do not be too concerned.