performance of car continuing to fade away

[richie]

I have asked this question before in the past about the performance of my car is not what it used to be.My stang for the last year has slowly been loosing power.Everytime I think I have found or fixed the problem I havent.I have replaced every ignition component except for the distributor itself.The fuel pressure is good and so is compression.I have also changed the fuel filter,plugs-autolite 24's,wires-taylor spiro pro.My gas mileage sucks to at 11mpg.i replaced the o2 sensors and added underdrives to hepl with mileage but they didn't help.The car doesn't have the power at all rpms,but it idles smooth and doesn't blow any smoke.I once did a leakdown test a few months ago but can't remember the exact results.They dial on the gauge was in the green low area.I hooked up a vacuum gauge to the intake and vacuum stays at a steady 17psi and goes up like it should when revving the engine up.The only things I think could be causing this loose of power are a bad fuel injector,clogged cats though the vacuun gauge was fine but that wasn't when driving.I also though maybe my valve seals are bad but the car doesn't smoke and its not a high rpm problem its at all rpms the car sucks.My stang is at least 10 miles per hour slower at any speed and in any gear.I am at a loose and need help.