this issue has been raised before too
one thing to look for is FPR (Fuel Pressure regulator), i am not that much mechanically minded as to how it works but i was reading a thread on Nissansilvia and a guy had the same issue with his CA18det which is prettty much a smaller version of RB's he changed his FPR and problem was solved. (however i would do the usual things first like clean AFM, throttle body, idle controller spring etc)
if i find the thread i'll link it
edit: heres the link
http://www.nissansilvia.com/forums/index.p...howtopic=398535