Everything you're suggest says inadequate spark.

IF...IF the phase sensor or anything like that was giving a problem, the chances of it being boost related are almost zero.

And it would probably manifest itself as something more substantial than a simple misfire....although it depends how aggressive or noticeable this is that you're seeing of course.

Closing the plug gap down to say 20thou is a cheap easy test

You say fuel is fine....but what are mixtures like leading up to and during these problems ?