Your fuel level sensor is going out, possibly some corrosion build up or resistance in the sensor/circuit back to your PCM. If you have any time to pull the fuel module out of the fuel tank, you can check the leveler (looks like a bar of soap with a metal rod going to a indicator level). See if you have any debris in the way.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille