You didn't list mileage nor when the last tune-up was peformed. As already mentioned, you first need to perform a tune-up. This include plugs, fuel filter, and air filter. The fuel filter can be changed in under 10 min. The HELM recommends a fuel filter change every 4/60,000. I usually change mine every 2/30,000, but recently I ran the fuel almost into E and changed it because I didn't want to take a chance on what sludge I might have picked up. Next, check for leaking injectors as well as any fuel line leaks. There are no Codes present (O2 and Injector failure should result in a code)? You could also take the vehicle to an emission testing facility to ID for ex. a restricted CAT. Your tire pressure is at spec? A/T fluid at spec? If the vehicle has approx. 100k miles, then I would have the top end cleaned. 3M sells a kit which uses an adapter to the fuel rail and the vehicle is run off the can until it stalls. If you use this, you should change your engine oil afterwards.
I buy Techron at WalMart.