Erratic Fuel level sensor
 

The problem is, when you first start the car, the fuel level is reading accurately. After driving the car for at least 30 minutes, the fuel gage slowly goes to empty. The low fuel warning light also activates.
The GM dealer replaced the fuel level sensor. The same problem remained and I returned it to the dealer. They said they repaired some frayed wiring close to the plug by the fuel tank. The same problem remained and I talked to the dealer, he says he will need to replace the entire wiring harness.
I tried adding the Chevron Techron Fuel System Cleaner, although I didn't believe it would help as the fuel level sensor was brand new. It didn't.

Any ideas are appreciated,
Thanks,
Dave