Any idea of what would cause an intermittent misfire in my cossie?

1st happened at pod, at high rev's full boost.

Now occasionally happeneing at low rev's with no boost. Today was at about 3 thousand. Rev counter flew up to 7 thousand but the engine rev's did not rise, scared every bird in my village with an almighty bang!

Starting to get really, really annoying now as it does not follow any pattern, does it anytime which is most inconvenient when playing with a Z4!!!!



Any advice would be much appreciated!!!