I was having the same problems with the 1080ti, I was getting 1850kh/s with one card but after some time it would drop down to 1550 and stay there. Sometimes it was a few hours and then drop and sometimes it was longer. It was suggested to me that my video card driver was crashing in the background and although it wasn't crashing the system, the overclock software was not able to reconnect to the card and so the card was running at base stats and not being overclocked even though the software showed it as overclocked.
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Interesting premise that I am trying out now. I rebooted the machine again, lowered the o/c from +140/+350 to +125/+300 and restarted hsrminer. Reported speed is now around 1280; let's see if it holds.
Welp, that didn't work... Exactly 1 hour after restarting at a lower o/c the hashrate dropped to ~1150 (instead of ~1170).
I've just started poking around in event viewer but haven't found anything yet on driver crashes. Not sure where to look, actually, but I'll figure it out eventually... I'm sure.
EDIT - actually, backing off the o/c may, indeed, have worked. I decided to reboot the machine and try +125/+300 again and it has been running fairly steady at 1220 kH/s for 6 hours now. I never did find the gpu driver crash in the logs, but that isn't saying much.