On my recently acquired 99 2.8 Coupe, I was getting a momentary lag when I'd pull out on the Interstate, mostly in 1st & 2nd gears. The "stumble" was increasing and it was when the car was also warmed up (Not the S52 M stumble).
Last night when I was getting gas, the gas pump nozzle stuck open and in a second, gas was overflowing and running down the side of my car. (F%#$?!!). The fuel pocket was completely full and some drained back into the tank. After that incident I would get a lag/stumble each time I pressed the accelerator pedal.
OK.. it must be a fuel supply problem-dirty fuel filter.
I installed a new fuel filter, fired it up and zero stumble, fast crisp acceleration. The car had 30K miles and the original fuel filter so for about $15 its worth the 20 minutes to swap it out.