You can try going even higher than 150. Maybe increasing this to 200 will allow you to reliably use a larger undervolt which can improve your full load temperatures. Using this feature to increase the lower end of the voltage curve can improve stability when undervolting.
Thanks for doing some testing. Very few people understand or use the mV Boost feature. I am always trying to sell and promote this useful feature.
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The Cache Ratio Min / Max variables control your cache speed. Another thought just popped into my head. Perhaps the default Min ratio of 8 is too low for some CPUs. Perhaps increasing the Min value might help improve stability. You could try increasing the Min to 16 or some other magic number.
As for the Max value, ideally you want to run the cache as fast as possible without it causing a BSOD. For some CPUs, reducing the Max value by 1 or 2 or 3 might be a way to improve stability without reducing performance any meaningful or noticeable amount.
Now that you have found a way to undervolt the cache without it crashing, you can try leaving the cache at -75 mV or -70 mV and try undervolting only the core some more to -100 mV or -125 mV. Is that stable? Does it offer any benefit such as a lower full load temperature compared to leaving these two voltages set equally? Using a slightly larger undervolt on the core compared to the cache used to be a sweet spot for some older Intel CPUs. For some CPUs, staggered undervolt requests do not accomplish anything. The CPU might decide to just ignore the higher core offset request value.
I have never played around with a 13900HX so I cannot say what will work best for your CPU. It is all trial and error. Have fun playing and post anything new you learn.