Regarding the non-hashing or zombie units - Still using cgminer 3.7.0, I have clocked them back to 300 mhz per unit and still there would be 1-2 non hashing units that would creep up after a few minutes from initialization. The units that stop hashing are physically random (eg: first run unit 9 and 11 would stop; or second run unit 2 and 8 would stop and so forth).
edit:
I've noticed that the unit goes bad due to a usb write error. Don't know if anything came across this usb issue?

Perhaps if there is someone who is running these units with no issues, are you using a usb hub? I'm thinking the hub might be the culprit. Using Dlink 7 port usb 2.0 hub > 5 units per hub.
I use this hub:
Tek Republic TUH-2700 USB 2.0 7 Port Hub with Power
http://www.newegg.com/Product/Product.aspx?Item=N82E16817659004I have one unit that did go zombie or just showed mhs instead of gh,just moved it around to another USB port & another 6pin cable.It hasn't acted up since.
Those same cables are on another miner & it works fine.................No rhyme or reason here,all my power & USB cables are the same,very strange...................