1999 Chevy S-10 Fuel gauge wanders even when vehicle sittin

The subject line pretty much says it all. You can be sitting in the vehicle perfectly still and the fuel gauge wanders all over the place. I have had another problem with the check engine light coming on when the vehicle is hot on hot days. I am told the computer code says it is the "evaporative emission sensor" in the fuel tank. But I don't see how the two could be related. Is it the dash gauge, the wiring, or the sending unit in the tank? Or something else?