What kind of gas are you using? I know that may sound funny, but switch to a different brand and see if that helps. I know that here in Oregon when the State mandated 10% ethanol, my cars all started getting 10-15% worse gas mileage and I can not run Arco with their blend. My cars all seem to like Chevron, 76 or Shell.

You may also want to try a good dose of Techron.

Good Luck,
Glenn