Deadly thing with 200 million year orbit keeps blasting Earth with death?

The great and minor extinctions in our fossil record have a bit of a rhythm near the 195 million year mark and another in the 26-35 million year range.   David Brin speculated in 1984 that the periodical encounter with a galactic shock wave or a ray-spewing black hole orbiting a little quicker than Earth's 240 million year track around the galaxy could explain this.  The math isn't much different than the calculations for earth-moon interactions.  If it exists, it is about 2.4 kilo-parsecs out from the galaxy core while earth sits at 10.  All this is in addition to meteor mashups.  A periodic particle flux can collapse Earth's ecosystem.  What a beautiful idea on a cosmic scale!

Round we go.

The author, David Brin, links to an update to this speculation which involves dark matter and the thoughts of NYU's Michael Rampino.