http://www.skylinesaustralia.com/forums/Ma...ai-t213473.html
it's been covered before mate. some simple answers to questions will help others narrow it down to.
have you serviced it recently? or is it due for one? if you haven't then maybe that should be the first thing to do before asking around what the diagnosis is. you'd be surprised how often a fresh change of oil, filters, sparks will solve many problems.