I filled up my tank Thursday night, and didn't go anywhere on Saturday. I thought it seemed odd I was down to a little over half a tank, then today on the 9 mile ride home from work, it went from one notch above half to one notch below. What the hell is going on?

Sounds like your instrument cluster is going... Could be a sending unit but sounds like the cluster. Are any of the other gauges acting up?

I wonder if the sending unit in the fuel tank is bad. I know that GM had problems with 99-01 or 02 where too much sulfur in the gas would corode the sensor causing it to have inaccurate readings. Don't know if this is your problem or not.

Do you keep track of your MPG?