Im not sure if you've managed to solve this problem yet, but it sounds like your igniter/coil packs could be on the way out.
My R32 GTS-T did a similar thing, i would be driving along, accelerate & the noise would increase, boost would build, but power just wasnt there, & you could feel the power dropping off as if there is no fuel there or something. Changed my igniter, & the coil packs for a set of split fires, & suddenly no more dramas. My car actually feels to get more top end grunt over 5000rpm, where as before it was dropping off from 5000rpm upwards.
Mine was only doing it on long drives where it was up at operating temp for half hour plus, but your igniter could be the problem here....hope this helps.