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.

I was running at maximum overclock and although it was stable with CCminer, it was not stable with HSRminer. I've resolved this issue by lowering the overclock just a bit to the point where the drops no longer occurred. I've also vastly increased the virtual memory and now I no longer have this drop in hashrate. I do not mess with -i or -c and so I can't confirm if any of that works, I can only tell you I was seeing the drops daily, and now I do not.

You can check your windows logs to see if the drivers are crashing, I've never checked the logs because once I made the changes it stopped happening. Hope this helps you guys..