What do the rpm's do when this happens? Pause, reduction? Any particular RPM?
If the car is going into limp mode I haven't seen anything like that posted yet.
The N20 and ZF 8HP trans do however have a known slight hesitation during hard accelerations but this is only a pause and this does not sound like what you are experiencing.
If the car is going into limp mode, be concerned that things are not right. If it's still going on after they said it's fixed drive it back in to the dealer.