Random overnight speed drop down to 5mhs on 1 gpu.

[Vwgtiturbo1]

Hi my first post on this forum and would like to say hello and probably bring a issue that I am sure someone heard or delt with before. I am running a 6 gpu rig ( xfx rx470) on a msi z170a pro carbon mobo on a windows 10 os with 4gb of ram and 120gb ssd. So yesterday with a friends help i was able to get 29mhs on all my gpus with hynix memory and they were running smoothly until i noticed this morning that one of my gpus dropped to 5mhs this morning. I shut down the system and unplugged it and restarted the rig with out it and thrn shut it down and plugged it back in thinking it will reset itself and it did no affect. Anyone have any suggestion on what I can try to get my gpu back up and running at its speeds. Thanks