SeanGoku

What happens with CPU usage during those dips? Since your GPU temps were moderate, it could be CPU temps causing the CPU to lag behind, or maybe some background process hogging the CPU. Windows update, maybe? I've heard Win 10 isn't very polite when it comes to timing its update installs. Since it stopped happening, it could have been that. In any case, I would suggest adding CPU usage to Afterburner's plots, so you can check that as well if it happens again.
 
That is to be expected, since the GPU was essentially idling. Only the dips while gaming are a concern.

That PSU isn't the greatest but if it was working before fine then there's no reason for it to not work anymore :/

Have you tried reinstalling your drivers?
Maybe up the voltage of the core by a few mV may help?
When the dips happen, do memory and core clocks dip with the GPU usage?

Hmmmm...have you tried using older drivers?

Anyway, I'm not too sure as the most common reason for clock speed issues is driver issues but...doesn't seem like the issue here :/
Do you perhaps have a second PSU you can try or even a second computer to rule out a GPU or a PSU problem? :D

Hi there SpaceGhostC2C.
I didn't see the CPU at the time the dips occured. You might have a point. And your right about the idling ofc. I was just up the wall at that point not even thinking straight anymore.
Anyway, I played another 3 games today and the problem didn't occur anymore.
I'm gonna just assume it was either a windows update that fked over my pc (thow  I never had such problems before, even during updates, as League of Legends uses close to no resources) or the latest update of the driver, that caused the problems every once in a while.
 
Thanks again to all who came here to help me.
 
Have a nice day.