G'day Norm,
That seems like it could be the problem and then as soon as I pull out an NGK plug it always puts doubts in my head. It is the correct plug for the motor. I have never really studied plugs to the point of the heat range numbers on the plug and how they rate the numbers between different brands. This plug is a BKR5E and the Champion is a XC12YC so do you think the Champion might be a hotter plug going by the numbers?
Nope, they'll be the same heat range - the plug makers all use different systems, for their heat range designation. For NGK's, lower number = hotter plug.
As for it being a faulty plug apparently this plug was recently replaced for the same reason, so I am expecting it to come back with the same problem shortly. This could also be an ongoing NGK problem so the next thing is to change brands and see how that performs
I'd think it unlikely - I've had much better experiences all round with NGK and Nippondenso plugs, in comparison with Champions.
Bosch plugs have been good too, except in LPG fuelled cars.
As nath says, it would be well worth having a look over the fuel system - I do know that uneven mixture distribution is a pretty common issue with any multi-cylinder engine that uses a single fuel metering device.
Whether that be a carby, or a single-point fuel injection system, such as the low-end EA Falcon 3.2 and 3.9L I6's had.