weird video issue with HS


2013 MBA i7, 8G.
this MBA is partitioned. Mavs on one side so that I can use ProTools10HD.
HS on the other side, my daily drive.

Every now and then, the HS part won't output video via Tbolt to my ext 24" Dell.
Tbolt goes thru a UA Apollo audio I/O 1st, then the Dell.
I've tried cycling the Dell, sign out/in of the MBA.
A pRam zap doesn't fix the issue.
Only running Onyx fixes the issue.
Kind of makes me think that there is a cache that's getting stuffed or corrupted or .....

This never happens on the Mavs side.

any ideas?