How does the wort taste post-boil but pre-fermentation? Next time, try saving some of the wort, and let it sit, without yeast, in the same chamber as the fermenting beer. Then taste it at the same time you taste the beer. Do you get a similar off-flavor?
Are you pitching fresh yeast every time, or are these repitches?
I recently had issues with wild yeast infections. Getting aggressive on sanitation of post-boil and repitch components (boiling ball valves and tubing, flaming the lip of the repitch vessel, etc.) seems to have helped.