Hey Tony.
Did you ever get an answer to this?

I have the same with my rig when using Windows.  (on windows i got blue screen thread stuck in device)
Swapped to ethOS and now just one of my GPU's stops hashing when overclocked.  So If i run them all on 30 mh/s one of them stops after 15-20 minutes (sometimes it can run for several hours).

I have not tried to clock the problem gpu to normal bios  or changed the clocks for 25 mh/s yet, due to I am not home the next 14 days