Silverado has reduced power, rough idle, and sputters?

Its almost as if its running out of gas... and has been for 1000 miles. When I press on the accelerator the rpms increase, stutters for a second, increase again, stutters. Its driveable, but obviously something is wrong. I figured I had some bad gas, but I've gone through 20 tanks full. A mechanic told me with a 2004 1500 5.3L Silverado the fuel filter is not likely to blame. I have new spark plugs. Any suggestions of where to begin?