Any help will be greatly appreciated.

My car runs sluggish almost all the time, haven't been able to figure what the problem might be.

From a complete stop it accelerates normally as it is picking up speed, but when the turbo gage reaches 12 o'clock it dies down as if it is gasping for air or chocking.

I floor the gas pedal and there's no immediate response; after a few seconds it finally engages momentarily and jerkingly (revs up to 3000 - 4000+ RPMs and the turbo gage is up to 2 o'clock).

I have to do this over and over in order to pick up some speed, once it picks up the speed it runs smoothly again (2500 RPMs) until it gets to the point again where the turbo needs to engage, and again, once it reaches the 12 o'clock mark I have to floor it again.

My mechanic has not been able to pin point the problem, he claims all the codes read normal.
I have replaced the fuel filter and the air filter; and have cleaned up the air flow meter. Just don't have a clue as to what to do next.

One thing I have noticed is that sometimes (not always), when I am at a traffic light waiting, I see white smoke bellowing out of the tail pipe (heavy at times).

Any clues?
Thank you in advance.