I have had some problems with diacetyl in both my lagers and a couple ales. I have a fridge with temp control which I use every time. For lagers I usually ferment at 50 degrees for 4 weeks (no diacetyl rest). In my ales I usually use WL002 and ferment at 65 or 66 then up to 68 after three days. If I am using a pilsner malt I do a 90 minute boil. I never leave my lid on or anything (until I am cooling).
Since getting my fridge it has been better but a couple beers since have still had the problem. Specifically, a Firestone DBA clone (from The Brewing Network CYBI), a Cali Common (from Brewing Classic Styles, fermented at 60 degrees w/ WL Cali Common yeast), and a Dopplebock (from Brewing Classic Styles). The first may have been fermentation temp swings (had problems with the controller), the other two were kept at ferm temps for about two weeks then moved into my "cellar" which is generally at 65 degrees.
Could it be under pitching yeast? I always make a starter (with a stir plate) based on Mr. Malty. Should I think about doing a diacetyl rest with my lagers?
Any help pointing me in places I could do better or think about would be great!