Were they iridium tipped plugs? They're such a delicate tip that it's not unknown for them to wear down and come off the plug... I'm sure I saw something about this in another thread.
Hopefully they were the wrong temperature range and they got cooked... and that it's not a fuelling issue and you've been leaning-out (as you suspect). - What boost are you running; do you have an uprated pump (I'm sure you've said before, but I forget)?