2000 Pontiac Bonneville hesitation

When driving in different weather conditions and speeds, my car hesitates. It almost feels like I'm running out of gas. I've had the fuel injectors professionally cleaned and the fuel filter changed hoping that would solve the problem. I've also had new plugs and wires installed. It doesn't seem to be a shifting issue and when I'm idleing there's no significate sputtering. Someone told me it may be the fuel pump getting ready to go but.
The car drive great except for this one issue. Also I've checked the gas mileage and I'm getting between 21-25 mpg in the city! There's no check engine lights on either. Do you know what could cause this hesitation?