Claymore’s version 12 is out now - try that instead. I could not get version 11 to be stable for more than an hour or so with RX480s, but version 12 running fine for last 15 hours or so.
I’m sure you’ve done this, but I found that upgrading version X to version Y with miners generally causes the new version to not work - and a reboot required to get any kind of stability. “stopping” the older version of the miner doesn’t seem to stop it cleanly - this goes for Claymore and Optiminer in my experience - but on restart, much better and no red lettering or other odd things.
It happens when you overclock or undervolt or both things altogether too much. Try relaxing the card a bit, don’t stress it so much. Bear in mind that even within the same model, not all the cards can admit the same OC/UV settings.