It turns out the issue was initially triggered by a Mod_Security limit, then a hard limit on the size of the /tmp folder. We created a different folder as host temp separate from Joomla tmp where we could set a higher limit and the issue cleared.
Make sure
1) max post and max upload are sufficient
2) make sure mod_security "SecResponseBodyLimit" is suficient
3) make sure your system temp folder is sufficient (default is /tmp)
We had two Acyba techs signed in to help us hunt this down.
