My specs are:
i7-6600k 4.5Ghz overclocked with 16 GB RAM
GTX 1080 Gigabyte G1 Gaming 2050Mhz overclock
Windows 10 (ver. 1607)
ASUS Z170 Motherboard
Dell S2716DG monitor with G-SYNC OFF at 144Hz
And my fps is as high as it can be in Diablo 3. In order to keep the GPU not too loud or hot, I limited FPS at 160.
It seems that if I compare my specs to yours, it's just the processor that is different.
So my questions will be:
1) What is the GPU/CPU usage in other games and in Diablo 3? Get CPUID and MSI Afterburner for that.
2) G-Sync on? Try turning it off.
3) Latest drivers clean install?
4) Update to Win10 or clean install of Win10?
You said - my OS is from my previous build - ??? You mean you had different motherboard / CPU / GPU and kept same Windows without reinstall? That's bad. I don't know whether it is the reason or not with Diablo 3, but it will definitely cause you problems.
5) Did you check your CPU overclock with the necessary tools to prevent throttling and to check if it's doing fine? If no, then reset CPU setting to default and try Diablo 3. Diablo 3 is more CPU bound then GPU bound, as far as I know
6) Is your GPU overclocked?
7) Try Diablo 3 in x64 and DirectX 11. Actually, don't just try it - it's how you should always run it.
Well, no other ideas. Hope it is of some help.