I have the exact same prblem with mine.... my CEL isn't on, replaced the fuel filter but no success... Whenever i have a full tank of gas, it doesn't sputter until i have less than 2/3 of a tank. I am at a complete loss as to what to do as well.

SeSportV 1999 Silver Contour SE Sport MTX (2.5) Air Intake Resonator Removed Silver Rear Valence Blue Turn Signals, Nozzles, Interior LED's Custom Blue Trim Single Silver Wiper (Bad A** and Unique) 15.9 1/4 mile @ 89 MPH