I have been trying to resolve this issue for weeks now and have not been able too. I have basically rebuild my PC over the last year and a half and will attach my components.
The problem is that with certain DirectX games; including, Crysis 2, Crysis 3, Far Cry 3, Unreal Tournament 3 Black Edition and PUBG, to name a few I notice the problem with, my PC's GPU usage drops during certain effects/areas taking place while my total CPU utilization is only around 50-60%. Furthermore, this happens with both my previous radeon R9 280 graphics card and my new RX 480 when I test; for example, Crysis 3 in the same area the game is limiting to 22fps when looking at that area giving only 20% GPU usage constantly, but when I test my RX 480 in one of our computers with an i5 4670k the problem is not present and my GPU is utilized 100% in that same area with at least 60fps. It seems to happen during explosions, or areas with heat and dust particle effects.
I do not believe my FX 8350 to be a bottleneck, since Crysis 3 gives my CPU a better benchmark score than the i5 4670k during the initial setup/benchmark in the console of the game and newer games such as Battlefield 4, Battlefield 1, Splinter Cell Blacklist, Call of Duty Black Ops 3 and Call of Duty Infinite Warfare utilizes my GPU at 100% with much higher frame rates constantly and it seems to use a bit more of the FX 8350 than the games I am experiencing problems with. I do believe it to be a driver/source-code problem between DirectX/AMD/Windows10 and unfortunately I am not able to test Windows 7 on my PC, since the PC with the i5 has Windows 7 installed on it. Doom 4 plays very well with Vulkan on my computer, but some other OpenGL and mantle games aren't loading textures and shadows properly.
In addition, I am regularly updating my graphics drivers(through clean installs with DDU), my Windows drivers and chipset drivers and the issue is still persistent. Also, my power settings is set to high performance and I have have monitored all the temperatures available through software and nothing is getting too hot with my CPU below 50C and my GPU(MSI RX 480 GAMING X 8G) around 75C; furthermore, the GPU usage drops only when I look at certain effects and it is not a stuttering effect like when certain components are getting to hot. My CPU and Graphics Cards run at the correct core clocks during all games it is only the GPU usage that drops which causes the GPU to use less power also( I have tried setting the power slider to maximum) since it "doesn't seem to need it" in those areas.
I would greatly appreciate help and if someone has experienced and resolved this issue before and if it is known that downgrading to Windows 7 resolves the GPU usage problems through source-code that might be missing with newer Windows versions. I have yet to test an NVIDIA card in my computer to see if it is possibly an AMD/DirectX/Windows10 combo issue. I don't know if AMD support is taking this to seriously since I might be one of the few with this hardware combo and experiencing this problem.