sls erratic/incorrect fuel gauge on occasion

I have a 1997 SLS with just under 102,000 miles. Recently the fuel gauge has been operating erratically and not reading the correct amount. I have kept track of the fuel used in the DIC, but after two weeks of city driving (about 220 miles) the gauge still inconsistently reads between 1/2 to 3/4 of a tank remaning. The DIC shows that 14.8 gallons of fuel have been burned. Normally at this point the gauge would read between 1/8 to 1/4 of a tank of fuel remaining. Also the last time I filled up the gauge would drop from full to empty in a matter of a couple of seconds, the chime would sound, the 'FUEL LEVEL LOW' message would appear on the DIC screen for about a second and then the needle would go back to full.

Any help is much appreciated!

-Brock