After several weeks of fruitlessly updating drivers and increasing BIOS cooling settings, I'm concluding that the 5i5RYH as designed is simply unusable.
Event at ambient room temperatures and the "cool" BIOS profile, the systems is consistently unstable with symptoms usually being that the WiFi adapter shows as unavailable in Windows, Bluetooth not working and after significant use of 3D graphics, often the system cannot even boot after restarting. All symptoms disappear after a while of letting the system cool.
If anyone has real solutions to these problems (which as written seem to all be related to overheating), I'm very interested. Please don't repeat the useless advice of updating drivers, changing operating system etc., there seems to be quite enough misinformation here.