It looks like I'm actually running into the issue described here: http://lists.puredata.info/pipermail/pd-dev/2013-06/019499.html
It would seem that the assessment there is correct: that pd is not correctly handling sync data, and a buffer is overflowing somewhere. Like the person who sent out that email, I haven't been able to get
portmidito build for OS X either, and so I haven't been able to attempt to rebuild pd with any kind of fix.
(Also, fwiw, the translation appears to be "Real-time Scheduling" )