Yesterday, I spoke with Nastia on Live Chat regarding an issue on a clients multisite, lunchsites.com, where the ste went down. The issue turned out to be a conflict in the Hummingbird cache. She talked us through getting the site back up and thought that the issue was resolved. Unfortunately, the site went down with the same error, again, this morning.
The same solution got the site back up (changing the 'WP_CACHE' setting in the wp-config file to 'false' but, clearly, the root casue hasn't been resolved.
I've granted Support Access to you to take a look. Just let me know if you need anything else.