So start with a deed cleaning with the canned air and check that all fans work like new.
The r9 380 was known to need it's heatsink compound replaced too. https://www.reddit.com/r/Amd/comments/4e9286/reapplied_thermal_paste_to_msi_390/?st=jcdodsd1&sh=02229302 is one of many articles about that.
Try it with the cover off too.
My computer will randomly turn off for no apparent reason, I originally thought it was my psu but I replaced that and it still turns off I also ran memtest86 and it didn't detect any errors I also don't think it would be my gpu but I'm not shutting out that idea.
I have found that my computer is more likely to turn off while having discord or a game running.
Does anyone have an answer to this or a watt of testing my cpu/motherboard/gpu/hard drive
Radeon r9 380
2x4gb corsair vengeance ram
Evga supernova 550 g3 psu
64 bit Windows 10
I'm not sure the brand of the motherboard