[Hummingbird] Hummingbird kills all css

With only browser caching and page caching it appears to lose css and functionality. The site has no effective layout but if I force a change of theme it sometimes works for a bit on the new theme. Switch back and it dies again. In live preview always as expected, just not on the live site. Cleared cache, changed settings, restarted apache, rebooted server etc but not effect.

Remove hummingbird and all works fine on all sites in the network.

Anything I am missing?