Soldato
Try disabling Cool and Quiet. This is only supposed to kick in when the CPU isn't under load but it's worth a try. A lot of people recommend disabling C1E power saving too.
My first guess is that on auto voltage it's supplying too much which is causing the CPU to throttle, then it's going back up to 4.2GHz and the same thing happens.
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Disable cool n quiet. Because your CPU is under load, this isn't causing your problem but it should be disabled anyway while you are messing with clocks.
Your CPU temperature isnt particularly high, so your throttling is most likely due to VRM's getting hot. Is the VRM heatsink hot after some stress testing? If it is, that's your problem. If the multiplier knocks down to 14.5 after just a minute or two of stress testing, there is not too much you can do about it but lower your voltage and hope that the CPU stays stable. If it takes a while to throttle, aiming some air flow towards the VRM heatsink might help.
If you have voltage set on auto, set it to manual and start with a voltage your motherboard autos at, this tends to be a little higher than it should be. So with a bit of luck you might be able to clock a multiplier or two higher without having to increase it to gain stability.
Does your core temperature shoot up to past 65/70 and then reduce after the multiplier changes to 14.5?
This will likely be throttling due to core temperature, meaning your voltage is too much for your CPU cooling. If this is not the case, and your CPU temperature stays in a safe range, it will likely be your VRMs. You should cautiously feel how hot they are straight after the multiplier is reduced.
The temps in the screen, seem quite high considering the clock is throttled.