Yosemite+Thunderbolt Display audio constant crackle

Constant crackle/skipping/breakup of audio on MacPro dual D700 + Thunderbolt 27". Problem also occurs with playback through Blackmagic device. Does not happen with internal MacPro speaker. Hideously awful problem, making our edit suite unusable.

Have tried all the usual suspects:

reset PRAM multiple times

killed coreaudiod through Terminal command line

force quit coreaudiod via Activity monitor

toggled "feedback on volume change"

multiple reboots

scrolled through Activity Monitor to look for unresponsive processes (none listed)

The last time we had this problem, with 10.10.4 (or maybe 10.10.3?) it was a quicktime helper that was not responding, and a driver update solved the problem. But we're running the current (approved for 10.10.5) Blackmagic driver at the moment, and all Apple software is up to date (except that we're still on Yosemite instead of El Capitan...)

Documentation of problem at

https://vimeo.com/164455726