Crummy fuel guage sending units. The GM trucks are notorious for that kind of thing. Solid vehicle mechanically otherwise though. The above answer is correct but to maybe help a little more, try adding some Techron to your gas tank. You can buy it at Walmart or Pep Boys or AZ. Some of todays fuels have a lot of sulpher content and this adhere's itself to the fuel sending unit. The Techron helps dissolve this. I own a 2004 Corvette and a lot of Vette owners were advised to try this to alleviate the fuel sending unti problem and it worked for a lot of us, NOT ALL but it may help. Then do a google search for TOP TIER GASOLINES or TOP TIER FUELS> There is a list of gasolines that have the right additives to help with this problem. I buy Conoco or Chevron or Texaco which already have the Techron additive in it so I don't need to buy it separately.