My Nx does something similar.. if im going like 20-30 mph and i decide to gun it it wont shift into the next gear until i let off on the gas a little bit. Since my trans. is electronic i'm guessing it might be a chipset issue. Other than that my transmission is fine, so I'm not worried.
I heard that the older electronic transmissions learned how you drove and the problem was that it learned its stuff during the break in period, so after that it kept shifting that same way. I know the automatic 300zx's had to be taken back to the dealer to be reset when such a problem occured. Who know's maybe time for a reset.