2001 Audi A6 Audi Engine performance problem nobody can di

Engine Performance problem
2001 Audi A6 6 cyl All Wheel Drive Manual

So my 2001 a6 2.7t has been acting up a little bit on me. Some of the things I have done to try to eliminate the problem is replacing MAF sensor, replacing Fuel pump, Spark plugs, FPR vacuum hose, oil, fuel filter, transmission fluid. And still it keeps happening. My car sometimes feels like it looses some boost I guess you could say durring early acceleration. When running fine I barely have to step on the gas for gear transitions and it catches quickly with very responsive acceleration, when running poorly I have to step on the gas a lot more so the car does not jerk badly durring transitions sometimes I can even press the pedal all the way down with not as much power. Also sometimes it feels like the gears on the shifter are way smoother on the good times. When the car does run good it is after I drive the car once and shut it off for a little while, then when I turn it on again walla. Any response would be greatly apreciated no import mechanic has been any help to me. I also thought it might be my fuel pressure regulator but I dont want to waste any more money.