I'm copying my thread straight from reddit (https://www.reddit.com/r/AMDHelp/comments/6ou6fc/asus_rx580t8g_strange_throttling/)
Computer Type: Custom Desktop
GPU: ASUS ROG-STRIX-RX580-T8G-GAMING 8GB GDDR5 @1451MHz core clock, 150% Power Target, 87°C Temp Target via ASUS GPU Tweak II
CPU: Ryzen 1600X with a Thermalright HR-02 Macho Rev. B @stock
Motherboard: ASUS PRIME B-350 PLUS
RAM: 2x 8gb Corsair cmk16gx4m2b3000c15 @2666 MHz
PSU: be quiet STRAIGHT POWER E9 | 480W CM
Operating System & Version: I.E. Windows 10N with all updates
GPU Drivers: Radeon Software Version 17.4.4
Chipset Drivers: shouldn't matter
Background Applications: shouldn't matter
Description of Original Problem: In some games I noticed framedrops from time to time. In Titanfall 2 it got so bad that I had <30fps for like ~1 minute but only a few times a day. So I went back to the monitoring & stability tools -> see Troubleshooting
Troubleshooting: I used GPU Tweak II to read out the clockrates and temps during gaming when the problem occured. The GPU clocked down to 300MHz - temps didn't matter.
Then I wanted to know when the GPU starts throttling, used Furmark and opened up my case to be sure everything has enough fresh air. Here are the results:
no throttling @65°C, 80% fan speed stopped after 10min
throttling @68°C, 60% fan speed after 3:15min
throttling @72°C, 50% fan speed after 2:25min
throttling @76°C, 42% fan speed after 2:05min
The GPU core temp doesn't seem to matter. My guess is that there is something else heating up and throttling but I don't know what.
CPU is out of question because it's cool enough and the RAM modules are also not affected by the GPU airflow. Maybe it's the mainboard? There is a passive cooler right next to the graphics card but this shouldn't happen right? It would be a big design flaw.