OK for starters readjusting the tappets is not a bad idea: http://www.nissanpatrol.com.au/forum...pet-Adjustment
Secondly.... For all money I would be searching High & low for a vacuum leak first up as the symptoms your describing sound very reminiscent of its effects, especially given the reactions to the 2x fuels.
If you've already checked for this then check again & be doubly sure before moving on to the next step.