I had a similar post to this however I seem to have narrowed down the problem, which is - My GPU's core clock fluctuates. But not on 60Hz only 144Hz and I have a sneaky suspicion that it might have something to do with fullscreen. I have the Crossfire icon overlay to tell me if CF is running in the game I play. In bordered mode I can see the FPS in Far Cry 4 main menu is around 2200FPS whereas when I go fullscreen it drops to 150-300FPS and this corresponds with the fluctuation reported in MSI Afterburner.
The temps are fine
It works perfectly sometimes and my core clock is a rock solid 1GHz. However, for the majority the GPU clock on both cards will fluctuate between 750MHz and 1GHz also the windows bar will flash through briefly.
Just to confirm, this just started happening as I got a 144Hz monitor.
Any help would be appreciated
I7 3770K @ Stock clocks / undervolted stable
Asus P8Z77 V LX MOBO
Corsair vengeance 16GB 1600MHz RAM
H80i closed loop water cooling for the CPU
2 x Sapphire R9 290's Tri-X
1TB Samsung HDD
250GB Samsung EVO SSD