well here's the problem everytime the gas gets below 1/2 the gauge goes up and down and is to hard to determine how much gas there actually is.. anyone know what's wrong?

 

9 times out of 10 this problem is always a fuel sending unit. It's a big problem in GM vehicles especially in the trucks. I think you can get just the fuel sending unit or you will have to replace the entire pump. Best bet is to call up the dealership and see what you can get and get some prices.