Hi, please fill out the system specifications details or give us a general idea of what you're using.
But to answer your question, the reason you are crashing when power saving is activated is because of the built in V/F curve.
When you set a negative -120 mV undervolt, you are setting it for all voltage/frequency curves.
If your system idles at 0.800 @ 800 MHz , it will now idle at 0.680 @ 800 MHz , which may not be enough to retain idle stability. There are portions of the CPU outside of the cores that have a minimum necessitated voltage, like the cache. The core voltage is shared with the cache (assuming you're on Intel). The cores might be stable at something like 0.680 , but the cache needs that higher voltage to function properly.
You can either set the performance plan to High Performance and leave it there - which has a tendency of locking your voltage and frequency at the highest setting.
Or you can set a static fixed voltage to assure it never goes above or below your target voltage (example: a target voltage of 1.10v - don't actually use this voltage. I don't know what hardware you're running. It is an example.)
The difference in idle power draw between a static fixed voltage and an adaptive offset is minimal (on modern CPU's), so long as your C-states are active.
I also recommend at the very least using Cinebench R15 and R23 to stress test. Ideally you would download OCCT (it's free) and run the built-in CPU stress test for 30 minutes, assuming all you do is game on said machine.
