gday mate...i had a vp that was doing the same thing...we couldnt work it out for the life of us....in the end i gave up and just learned to live with it...
but heres a list of things i think it could be...
spark plugs,
leads,
Coils,
dfi module,
fuel filter,
fuel pump,
valves,
valve springs,
ecu,
Fuel regulator,
O2 Sensor,
Injectors,
O Rings,
MAP Sensor,
thats all i can think of off the top of my head...hopefully this helps you out.