I'm a moderately experienced homebrewer, and recently I've been making a lot of imperial stouts--variations on the same recipe, hoping to dial it in. Decently strong ones: my OG is usually around 1.111, and I finish between 1.030 and 1.037 (I'm aiming for a sweeter stout, so that's fine). I pitch 2 rehydrated packs of Safale S-04 (new) per 5 gallon batch. I primary until it's done fermenting actively, transfer to a keg for secondary, and let it age for 1-2 months before crashing in the cooler.
However, I have a persistent alcoholic/solventy flavor when I first try the beers. This has happened 3 times now. Really hits you in the back of the nose when you sip/inhale over the beer. The rest of the flavors are fine; no weird yeast making nasty tastes.
I know that this is typically a fermentation temperature issue, but the odd thing is that, if anything, most of these beers ferment a little low (I have a poorly heated room for fermentation in the winter, so even with a water bath, the temps tend to stick between 58F and 63F).
What could be causing these alcoholic/solventy flavors? Should I lower the primary temperature still more?
Also, I've been taking the beer out of the water bath when it's done with primary and letting it secondary in the air. This might lead the temperatures to get somewhat higher during the days (though never more to 70F, and typically not for more than an hour or two). Could this be when/where the off flavor develops?
Could this be a symptom of stressed yeast? Should I pitch more/re-pitch during fermentation? Alternatively, is this a typical "young beer" problem with strong stouts, and one that will fade with time?