Your rig is underpowered.
I suggest running six cards see if the six do 27-28 hash.
The psu is 1000 watter and even with under clock 🕰 I think you are aging the psu. As most likely you are using over 720 watts 24/7/365. Atx rigs do not like to do that.
It then short changes the mobo and your weakest card suffers for it.
See if the card works better if you only run six or five cards on the mobo.
I believe this is not the problem as the Rig is consuming 650 watts in total.
I've tested it with less gpus and the problem persists. I am going to test this gpu on another computer ... I think she has a problem.
And again, i don´t know why people can´t give us all relevant informations but want help
DRIVER (version), OS (and version), OC settings (POWERLIMIT, CORE CLOCK, MEMCLOCK), TEMPS, HOW YOU OC, WHAT ARE THE CLOCK`S FROM EVERY WHEN YOU ARE MINING, VMEMORY MANUFACTURER from every card, MONITOR PLUGIN IN IGPU OR 1660?
Here is some additional information:
OS: Windows 10 PRO Version 1709.
MONITOR PLUGIN IN GPU0 1660.
I would like to remind you that this rig worked perfectly for months and the problem appeared abruptly.