Hi Dan,
Normally when the RPM glitches the problem would be caused from the cam sensor. Intermittant faults are not uncommon with these sensors, but I wouldn't order anything until the problem was diagnosed. If you're in Sydney we have a scan tool at our workshop that can read the codes off the factory Nissan ecus which does solve a lot of problems quickly. It also has other diagnostic functions for identifing fuel, ignition and timing problems down to each cylinder.
Regards,
Jeremy
Tweakit Performance
http://www.tweakit.net/