1999 SAAB 9-3 repair Question Fuel Pump problem?

Hello,
I have a 99 Saab 9-3 convertible that I bought a month ago. I noticed that the power output was a bit sub par when I was going to buy it but I also saw that the tranny fluid, along with engine oil was in desperate need of a change. GAve the car a nice tune up, and flushed all fluids. Got a bit better, but the car is still not as responsive as it should be. I'm getting about 10mpg less than I should be. At first when I start it up, it will seem fine and relatively responsive, but the more I drive it the less responsive and powerful the engine seems. After a long drive, (about 25-30 miles) I will find myslelf pushing the gas pedal down almost all the way and the car barely accelerating. I made a trip from San Diego to Ramona, and towards the end my car wouldn't go past 55-60mph.
I think it is the fuel pump, because of the mpg and less responsiveness, though I also think it may be O2 sensor. I know its not the transmission because the rpms seem normal and the car doesn't really struggle or anything.
Any input. Thanks a bunch in advance!
Ryan