It's funny you are getting this issue. Because on my main PC with 2 GPUs I've tried mining ETH and very randomly the system would just freeze. Either once a day or once a week.
Tried ZEC mining and never had this issue.
I think it's maybe the higher memory clock that makes the system unstable from time to time. Only solution I can think of.
Yeah its annoying isn't it. Funny thing is, even with NO overclock... ETH still crashes it
You got to live like this with AMD cards until you find their sweet spot. Even if you have no overclock settings, one possible cause of crashes would be an incorrect BIOS mod. Try the polaris bios editor with the one click patch thingy.
If it still doesn't work try changing your risers! Good luck matey