1a. Use BFGminer 3.99 with Gridseed support for Windows on 1 or all your miners.
1b. Scrypt only at 800.
2. Let it run on a pool that does not switch coins. Run for at least 1 hour then look at the Hardware errors (HW). In the Manage Devices, you can see which chip on which miner is giving you the hardware errors. Make note of each miner's ID and Chip ID that has the errors. Stop BFGminer and reset your miners for a few seconds (unplug and power off). Turn them all back on and reconnect.
3. Restart BFGminer 3.99 with 25 steps lower (in the first test of 800 minus 25 would be 775 for the second test so on and so forth until no HW errors exist).
4. Run for at least an hour (same pool as before) and make note of each device and any Hardware errors on which chips (if any).

Repeat steps 2, 3 and 4 until you have no errors on any chips on any device. You should now have a detailed list of your max settings for each unit you have. You can then run CGminer with each individual unit's max setting.
You may be able to do the diagnostic with CGminer also but I've only worked with BFGminer since day 1 of mining.

I have the hardware ID on each miner so I know which one is which rather than by com ports.
Also make sure the USB plugs are snug and flush on both the miners and hub/interface. If it wiggles around in the port, you can bet you won't get any good results from it.
And if you do have a defective unit, you now know which one it is and have test results to back up your claim.
FYI, I have some in the 400's and some in the 290's on 850. And not a single HW error after 6 hours of mining.
I plan on looking at the ones in the 290's to see what chips are under performing and disabling them or upping the frequencies on those to see if it jolts it back to where it should be.