Why does the gas gauge on a 99 Alero suddenly drop to Empty even though the tank is completely full How do you rectify the situation?

My 2000 GLS does the same thing. When I fill it it reads full for a few moments then goes to straight to empty. If I am going down the road I can shift into neutral shut the motor off and restart but the gauge does the exact same thing again. I was told it was the sending unit but have not fixed it yet. The guage works fine after some driving and the needle comes off full.