ian.egerton,
There are several variables and possibilities to consider with respect to Core temperatures and stability.
Although Throttle temperature is 100°C, Core temperatures above 85°C are not recommended. Core temperatures below 80°C are ideal.
The Arctic Cooler i30 CO is barely adequate for the i7-8700K at stock, but not when overclocked or when Package Power is as high as 160 Watts. Although transcoding software such as HandBrake runs AVX code, the peak fluctuating workload and power consumption typically shouldn't exceed the steady-state true 100% workload of Prime95 Small FFTs with all AVX test selections disabled.
If Core voltage at 100% workload is only 1.23 then processor degradation is
extremely unlikely, which typically doesn't begin to occur on 14nm processors until Vcore exceeds 1.4 as
Phaaze88 has pointed out. However,
motherboard VRM degradation is always possible, which can cause unstable power delivery to the CPU power plane resulting in intermittent instabilities and BSODs, especially when lowest Vcore sags occur during highest peak workloads, as
keith12 has pointed out with respect to LLC.
You may be experiencing memory instability, which is often difficult to isolate from CPU instability. Record the error message(s) and code(s) when BSODs occur. Use a pink eraser to clean each module's edge connector contacts, then reseat and / or swap modules to different slots. You can increasing memory voltage in 10 millivolt (0.010) increments to see if your rig stabilizes. It would also be prudent to run MemTest86 as well as Windows Memory Diagnostic Tool.
TIM (paste) degradation is most obvious at 100% workload due to high power and thermal dissipation, but is much less noticeable at idle due to low power and thermal dissipation. This also applies to Intel's internal paste between the Die and the Integrated Heat Spreader (IHS), which also degrades over time. In such scenarios, the only solution is delidding.
Complaints about high idle temperatures on various websites and forums are commonly made by users who are highly inconsistent about what the term "idle" actually means.
Idle means just 1 or 2% CPU Utilization in Windows Task Manager;
not running a light workload and surfing with various browser tabs open while streaming, etc. Unnecessary background Processes and Services account for spikes, fluctuations and high idle temperatures, especially when combined with high ambient temperature.
Most users don't mention ambient temperature which can be a huge variable. Many aren't accustomed to monitoring thermal behavior, so Core temperatures may go unobserved for countless months. Seasonal indoor ambient temperatures might be 10°C near the Arctic Circle, or 40°C without A/C near the Equator, which equally all computer temperatures, whether at idle or under 100% workload.
The International Standard for "normal" room temperature is 22°C or 72°F. So when providing full system specs, it always helps to include ambient temperature so we know whether or not your rig is running in a reasonably normal environment.
CT