I quote myself to bump my post.
Do anyone have the same issue ? I've been surfing forums all around and can't find anyone with this issue.
Do anyone have any clue about what can be happening ?
Thanks a lot.
My guess is, that this is related to GPU's power levels. I use GTX 970 on Ubuntu 16.04 and it has four power levels 0..3. When I'm mining, it spontaneously goes to level 2, but I can force it to level 3, and it gives slightly better hashrate with it.
Check GPU's power level, if it changes as your hashrate changes.
Thanks for your answer. I will walk this road and let you know what happens.