Gremlins have made it down under -

[circmand]

Could be a vacuum leak around the intake manifold. Using a stethoscope, a piece of garden hose or some such listening device might help you locate a leak if you poke around with it a bit when the engine is running. Another quick test would be to spray carb cleaner or starting fluid (ether) around the base of the intake manifold while the engine is running at any stable idle. An increase in RPMs would tell you that the spray is near the leak.

I would also say vacuum leak. Note if using a hose to hear the leak remember to empty the water out of it first