[Gaz64]

Greg, I work For Holden, I have a few questions and hopefully will be able to help you.

What ECM are you using, factory or aftermarket?

If it's factory are you using the AFM or has it got a mafless tune?

While searching for a misfire/flatspot etc, you should not disconnect the sparkplugs with the engine running.
What happens is the other three on that bank will go richer to compensate for the high oxygen content emitted from the cylinder you cause the misfire on. Although unburnt fuel comes out the exhaust port, it's the oxygen we haven't used that causes the others to go rich.

Have you put TECH 2 on yet to see what codes (if any) are stored.

Lean bank DTCs P0170, P0171 are common and PCV ventilation hose or loose intake gaskets are common.

If you can drive through the flat spot and it makes good power, then you can count the fuel system and the igntion system out. I'd say you might have a fuel trim issue, look at data display STFT and LTFT.