Ian.
I'd be happy to give you the benifit of my experiance but your not answering my questions
I suspect you need to run a colder plug, but that just a quess, not knowing what it looks like

Does your engine seam to have allot of compression?
What doest the plugs filiment look like when it comes out of the engine?
Do you have a temp gun? what's the engine running at?

Try running a cold plug, McCoy mc-9 or OS A5 or R5

OS chart


McCoy chart