I'd vote for checking the Catalytic (is that how itÆs spelled?) converter.
Mine was plugged about 6 years ago and I could only get about 200 miles per tank. It eventually caught fire. Luckily it was a typical Seattle Autumn day (pouring rain) and I was able to get the fire out quickly and it only burned some of the rubber exhaust hangers. After I got the vehicle home, I took the Cat off and it was melted almost completely solid inside. After I "fixed" it and put it back in, the mileage shot through the roof, and the power increased considerably to boot.