Is "unstable" clock harmful?

[xinxilas]

Hello friends!  !!

I have 3 1070, and 2 1080

Here on my little-rig i has 4 of them.
https://www.dropbox.com/s/kwz2chdpkcem4di/clocks.jpg?raw=1

Unfortunately my 1070s are weak, if i get < 66 % Power Limit, they run at 900mhz Sad
They just run at > 1400 mhz @ 65% TDP,  if i overlcock for at least +160 on Core Clock
(by doing that, they can reach 31mhz on ethash)

But the Clock became pretty much unstable, core drops and raises for example between 1380 and 1430 every second

Look above my 1080, it runs perfectly

My 1070s, get good results in my opinion, 31mhz at 97W(by the way, is it a good result for an 1070?)

Any help?!

[xinxilas]

any tip?