What hub are you running it from? I only see this on hubs below 2A or cgminer 4.2.3. If you have min 2A hub you should get around 12.5GH even over 24hrs. Just kicked off 24 hrs test  to see what I get on my hubs. Also why are you getting HW errors on cores? I get none (maybe 1) without OC.

I have been testing different versions of cgminer and I found that 4.01 works almost flawlessly. On 4.2.3 I get usb errors and random crashes, and slower speed. Any idea why 4.0.1 works so well but not the new version?