for me it was just a matter of eliminating other USB drivers and devices. Started with the minimum (say, one fader pack) and ensure that you can get C7, your audio device and one fader pack working. Obviously ensuring you have the latest drivers for everything. Add in device by device (after reboot would be wise) until you find the offending HW and driver.
I had to replace some suspect belkin USB repeaters and finally found that my problems occurred after removing (powering off) a certain piece of hardware (My Fractal AxeFX II). Interestingly, my Yamaha MoX8 keyboard can be switched on and off with no impact on the CMC stability.
I've just learned to live with keeping my AxeFX disconnected unless I need to do editing or firmware upgrades.
Once you know the source of your problem, you should be able to find a work around.