My 3.0 manual 53 reg Z4 is doing a silimar thing.
I would describe it as hesitant acceleration rather than fluffy ;-)
A few months ago i had a coil pack fail and i was told the others usually follow suit once one goes.
I recently noticed this hesitation when accelerating from a low speed and low revs, usually in 3rd gear.
It's fine most of the time though.
Mine has covered 45,000 miles and its due for an oil change in another 1,000 miles or so.
Having read around people mention coil packs, plugs and fuel filters...
I might just get the plugs replaced seeing as i've got new coil packs in it now..