How's your spoke tension? Certainly drums like the EW design are susceptible to non-uniform spoke tension. Another possibility is that the drum has a "woof" from some sort of mischief in the past.
It wouldn't surprise me if Douglas machined their brake drums after the wheels were built, so in response to "how accurate are the drums?" I'd guess "pretty accurate". That said, 0.050" is a lot to machine out of a brake drum, and I'd experiment with spoke tension first.