2001 AUM Golf Gti 1.8t, just had the lambda changed as it was bank 1 running rich, and new coilpack, but i'm still getting a problem when accelerating normally, most notably obvious in first gear

When you accelerate normally, (not hard, but not really gently) at about 2000 rpm, acceleration slows, and then picks up again. it is enough to throw you forward a little in the seat (e.g whilst leaning on the backrest, if the acceleration slows, you move forward off the backrest of the seat a little) it doesnt do it everytime, but happens quite frequently. can be quite embarressing with passengers when they are thrown foward a little :embarassed:
it is also noticable in second gear too at a similar sort of rev range, but just not as much throwing you forward. I though this could have been due to the overfuelling, but obviously not. There arent any other mechanical errors showing on diagnostic check. could this be a slightly faulty MAF, or a dirty throttle body, maybe something else, or might it be the fact that an oil breather has split? The turbo is also on its way out as the seals are leaking, and there is some play in the shaft, but it doesnt happen anywhere near when it starts boosting, so i wouldnt have thought it was anything to do with the turbo. Does anyone have any suggestions/help/advice?
Cheers
