XTU may be interfering with the Gaming Center resulting in the crashes, as it too uses the XTU service. Better to use ThrottleStop with a delayed start up (2 min) IMO. Undervolting the CPU you have very little to lose, possibly a lot to gain. You can also reduce the Turbo Ratio Limits in ThrottleStop as unless the game is CPU bound, few need such performance as the 8750H offers. Better to establish a stable undervolt first.
I've an ROG GL703GS and it hardly reaches 85C when fully loaded, with a -140mV undervolt (CPU in isolation 75C). Your going to need to spend some time on the ThrottleStop forums, although the basics are easy to pick up. As long as you set a stable undervolt the system will not crash, BSOD etc.
Match Core & Cache offset values, say starting at -80mV and drop until you reach the point of stability. ThrottleStop has a built in test (TS bench) if your seeing any errors in TS bench (1024M) the undervolt is too deep, the Core & Cache voltage needs to be increased. I use my systems for work purpose I generally find lowest value then come back up 10mV to ensure stability.
Each CPU is unique, so there's no way to present an optimal value. My GL703GS can go as deep as -153mV, although to ensure stability I run it with an offset of -140mV it has yet to crash or BSOD etc. in over six months of heavy use, on all performance profiles or battery.
Doubts? -140mV undervolt,Turbo limits unlocked, CPU PL-1 reset to 65W, default being 45W. Under Prime95 Small FFT stress test CPU remains to be in limits with no thermal throttling...
This is a highly unrealistic scenario, with a massive synthetic load applied to the CPU. You will never push the CPU to such extremes while gaming, at default the 45W PL-1 limit will cut in at the 28 second point.
CPU on my GL703GS levels out at mid 70C as can be seen below with default Turbo limits, with -140mV undervolt.
Cinebench R15 & Prime95 running
CineBench R15 Undervolt with the default 45W PL-1 limit in place, you can expect results similar to this, which is strong for the 8750H, newer chipsets can push as high as 1300CB
FWIW there's nothing wrong with the notebook, equally some can be done to reduce the temperature, which is always desirable. ThrottleStop is a powerful tool and you should take it step at a time with understanding of the application. XTU also has interaction with BIOS which I don't care for, while ThrottleStop is simply an application that can be easily reset/removed.
GL703GS, 32GB, GTX1070, NVMe & SSHD, W10 Pro 1809 - ambient temp approx. 25C, stock paste