Were the injectors flow tested and matched?

What's the fuel pressure doing under boost?

What condition is the ingition system in?

From my experience with modified turbo's (I used to own a modified Liberty RS turbo running 25 psi and a number of 13b turbo rotaries) I'd say there is a weak link in the ignition somewhere. I'd be looking at the coil(s) and leads first.

It could be caused by the ignition breaking down under boost, excessive fuel pressure or a combination of these.

Those NGk's hate rich mixtures and if given the opportunity to foul they don't recover.

That #6 plug definately wouldn't be firing under boost now in that condition that's for sure!

let us know how it goes.

Ross