Check the ground near the front of the tank near the rearend. A bad ground there will cause your problem. If you need to replace the sender, I STRONGLY recommend the pricey GM sender ($200) over the cheap import ($59). I see at least 1/2 of the cheap ones fail. Always test your new sender with an ohm meter (your old one too) by hooking up your meter & checking that it reads zero @ empty and 90 at full.
I know the GM unit is expensive, but it's worth it!! I swear the aftermarket senders are allergic to gas..
