Originally Posted by DazC

It could be lean. You nead an AFR gauge on it to make sure you're around 11.9 to 11.7:1 AFR on boost. You could also be massively rich. If you're straying into the 9 point someting to one area, then you will be getting rich misfires.

If you are fueling ok on boost, then check the valve springs/cam lobes. I have had instances of weak valve springs being forced open under boost.

Also set your plug gaps to around 0.6mm to make sure you're not suffering ignition blow out.