What type of yeast do you use? Some English yeasts produce a lot of diacetyl.
+1 - very common with many English strains. If you are getting diacetyl from WLP002 - well, yeah, the yeast is the problem. WLP001 - 'nother story.
Another thing that was not mentioned is that aeration during transferring can cause diacetyl to re-form. I can't remember exactly why this is but it can be an oxidation problem post fermentation. Or so I've been told.