Sounds like the fuel sending unit is going bad. On occasion, some have had theirs straighten around on its own after a time. GM blames it on the sulfur content in some gasolines.
You can replace the fuel sending unit or you can try the additive they have listed in this TSB first to see if it helps.
Fuel System - Fuel System Treatment PLUS(R)
Bulletin No.: 05-00-89-078
Date: November 09, 2005
GM Fuel System Treatment PLUS, Fuel Sending Unit Corrosion and Fuel System Deposits
2006 and Prior All GM Cars and Trucks
2003-2006 HUMMER H2
2006 HUMMER H3
2006 and Prior Saturn (Canada Only)
2005-2006 Saab 9-7X
GM Fuel System Treatment PLUS
GM Fuel System Treatment PLUS, P/N 88861011 (for U.*. ACDelco(R), use 88861013) (in Canada, 88861012), is now available.
Added Benefits and Uses
The PLUS portion of GM Fuel System Treatment PLUS is the addition of a filmer additive that, when used regularly, can protect fuel system sending units from the corrosive effects of certain sulfur contaminants found in some of today'* gasoline.
Sulfur contamination can disrupt electrical continuity of certain fuel sending units and lead to erratic or false fuel gauge readings. With scheduled usage, GM Fuel System Treatment PLUS protects against the effects of harmful sulfurs in gasoline.
You Get Four Benefits
One bottle of GM Fuel System Treatment PLUS added to the fuel tank at each oil change:
^ Cleans - Sulfur corrosion from fuel gauge sending units.
^ Prevents - Harmful sulfur components from attacking sensitive fuel system electronics.
^ Protects - By coating metallic surfaces of the fuel system.
^ Removes - Engine deposits left from use of poor quality fuels.
GM Fuel System Treatment PLUS will replace the original GM Fuel System Treatment, P/N 12345104 (in Canada, 89020804). The original formula may still be available for a limited time, and provides the excellent Fuel System Cleaning aspects of the new product without the filmer.
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