I have found a thread with the same problem
http://300zx.co.uk/forums/showthread.php?t=124009&highlight=automatic+gearbox+problem
Were do i go from here does the box have its own ecu? if so how can it be tested?
it is very powerful 1 and 2nd are perfect but as soon as it should be changing into 3rd it sounds like its hitting neutural and changing back down but just revs high and redlines