im having a similar issue at the moment, but i'd describe it more as being a bit rough.
i half put this down to only using my car for short journeys of late, so this morning took it for a bit of a blast on the motorway and really put my foot down. can't say its made much difference to be honest
also have tried this bg244 fuel additive that is mentioned on another thread, which is supposed to just clean the engine through a bit. have a read up on it. i'll let you know how i've got on
my 159 has 103k on the cllock now, so is ticking on a bit, but still a pleasure to drive ;0