Dropping hash randomly

Hello, i've been mining for quite long time now, i'm using claymore's zcash miner right now and I have a problem. Randomly my hashrate is going down to about 50% of original one.
I'm mining on R9 280X, my avg hashrate is 280-300.
I don't think that drops are related to any user activity on computer, because i was trying to catch moment when it drops, once it was while watching a movie (after like 80 minutes), once while browsing twitter, but sometimes it even drops while computer is idling at night.
Every program that could potentially use gpu in the background is turned off, i updated drivers, i don't use antivirus.
OS - Windows 10
I was looking at core and memory clocks, it's all normal

Plus, if i'm playing a game or doing something that uses most of gpu power, and after that i'm starting miner it's automatically starting at 150 hashrate and never goes up

The only solution i found to fix this is hard reset (soft sometimes doesn't work for some reason)

But i'm getting sick of restarting my pc every time it drops, so is there anyone who had the same problem or know how to fix it?

I have the same problem with an R9 270. Get 190 h/s normally, get random drops to 130, 100, or 70.