Everything is trial and error. Not a lot of people are trying to undervolt with Undervolt Protection enabled.
In ThrottleStop, open the V/F Point window and try setting V/F Point 1 to 150. Do the same for both the core and the cache.
Fill in all four profiles if you use more than one profile. I am a single profile kind of guy.
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Are you still using vastly different values for the core and the P cache offsets? Try setting those two offset voltages equally. See if you are game stable at -50 mV for both. If yes, go for -75 mV and then try -100 mV.