Lately I've been noticing a fair bit of variability in my bottle conditioning. Some beers will go way over-carbonated, some not. I would love help thinking through the sources of variability here, in case I'm missing something critical.
I routinely check the gravity of my beer as it is fermenting, and usually let it sit for 1-2 weeks after it stops dropping in gravity. I never bottle anything that's above 1.018, unless it started pretty high to begin with.
Using the nomograph from Palmer's book, I should be dosing my beer typically with 0.7 - 0.8 oz/gal of corn sugar to hit 2.25 volumes. I usually stick with about 0.6 oz/gal (3 oz in 5 gal).
I bottle in clean 22oz glass bottles, let the beer sit at 63F for a few weeks (usually 3 weeks or so), and then when everything seems good I put the batch in my refrigerator (converted chest freezer, actually) and hold it at 33F indefinitely. Sometimes I'll have too much beer and move stuff out of the fridge back into the warmer 63F basement. I am pretty fastidious about sanitization, I use a lot of star-san in the obvious places.
There are definitely beers that come out of this process (not all of them, and as far as I can tell it's not even every beer in the batch) where after a few months, they're crazy. Pop the top and expect a lot of foamsplosions. I thought at first that maybe I was having a sanitization problem with gusher infections, but I have since read that (A) gushers make the remaining beer taste bad and (B) they're likely to be a whole-batch problem stemming from infection in the lines or bottling setup.
I'm wondering if maybe I'm not getting an even distribution of sugar in the priming bucket? I pour the sugarwater into the bucket, then rack the beer into it. I don't stir, as I try to omit anything that would add a risk of infection.
My inclination is that this probably happens more often with my WL002 beers, the stouts and browns I brew. At first I thought it was something about residual long-fermenting sugars in dark malts that the yeast were nomming on over time, but maybe it's the yeast? I have a similar problem with WL380, a hefeweizen. Starts out fantastic, but then after two extra months in the bottle, half of them are waaaay overcarbonated.
Maybe the yeast is floccing out too soon, and the disturbance from transferring to the bottling bucket reawakens it? Unless my math is off, 2oz of sugar amounts to about one gravity point in 5 gallons (44 ppg for corn sugar, 16 oz per pound, 5 gallons), so if the yeast even have one more point of fermentation left in them when they go to sleep, that'd throw off the priming calculations something fierce.
Ideas? Direction? Maybe I should bite the bullet and force-carbonate instead. At least then I could use a plate filter or something to get the yeast out of there.