I tried mining Zcash getting more or less 290 sols/card without any tweaking and it's stable. I Let it run for a day and not a single problem.
when I tried eth and hashrate was dropping and finaly crashing, i was at PL 70 core -200, -100, +100 (tried a few to see if it changed anything, answer is not from what i saw) and mem+500.
I tried
+100 +800 = crash
-200 +600 = crash
anytime i try to go above +500, it seems it's not stable and eventualy crashes (real pain in the ass, btw)
Since i got no problem whatsoever with PSU when running Zcash, i guess it must be the GPU/drivers that are the issue?
Even with hynix mem i should be able to OC them to get 22Mh on eth i think.
I had the same issue, crashing because of nvlddmkm after mining for a few hours. the nvOC linux helped with that issue for me