When Stress testing, what is the temp of your CPU? Anyway, Disable the automatic restart so as to confirm that the restart wasn't because of a BSOD. You've already undervolted your CPU btw.

Just because the laptop's fan is producing a loud sound, doesn't mean that the cooling system is effective or efficient. Do a comparison between an undervolted state and a stock state to see if there really is a difference. HWmonitor is really good for recording down your max temps.

Stress testing is done for stability checking, it's fine if you don't do it, but there might be a chance that you've undervolted the CPU a tad too much and cause the system to crash. Another thing, how long is 'half-way'?

On a side note, disassemble the system and blow out the vents as well as reapply the thermal paste. Maybe that will solve your problem.