My pc: i7 4790k, MSI r9 390 8GB, 16GB DDR3-1600 Crucial Ballistix Tactical, GA-Z97X-SLI motherboard, BenQ XL2730Z 1440p 144Hz monitor, Windows 7 Ultimate, 16.6.1 Crimson Drivers. Everything runs on stock speed, nothing is oveclocked.
Sometimes the card can't keep up the clock speeds when it is needed. It only happens when I play games in windowed mode, browsing in Firefox or watching movies. I am getting horrible stuttering when scrolling in firefox (smooth scrolling enabled) or during video playback, or low fps in games. It seems that driver doesn't recognize that heavy 3D application is loaded, when it is loaded in windowed mode (or borderless windowed mode). For example, I load World of Warcraft in borderless windowed mode, horrible stuttering, fps is around 40fps, core clock speed is around 500-600MHz. I switch to full screen mode and instantly everything is smooth as silk, fps is 100+, core clock is 1040Mhz as it should be.
I have ULPS disabled (although I heard it only affects SLI configurations but I disabled it anyway), Power Efficiency is off and I set Power Limit in Overdrive to 50% in Crimson settings, nothing helps. The only way to fix this is start a game in full screen mode for a couple of minutes, this will "kick start" my graphic card for some time, or reboot the pc. I tried old non crimson drivers (15.11.1 and 15.10), but this bug happens there as well, but less often I think.