Are the gpu's overclocked?

first step would be set them back to factory.

I wouldn't recommend nicehash legacy.

use claymore and lower the intensity   ( -i 4,4,6,6,6,5,6 )

if the problem persists get a USB watchDOG and just set the miner to start on startup and live with it, it will just reboot when theres a problem and start mining , you wont miss much.

https://www.ebay.co.uk/itm/Hardware-WatchDog-PC-USB-For-Mining-Unattended-Operation-Crash-Auto-Recover/263151123219?epid=2181387617&hash=item3d45077b13:g:2mEAAOSwYeRZ8FMm