I own a MSI B450 Tomahawk MAX motherboard before and had the exact same issue. The VRM on this motherboard are bad quality and is almost broken.
Due the higher temperatures in your case caused by the new graphics card your system crashes.
A workaround for now to disable disable PBO or put the processor in 65W eco mode. Do you have a lower end AM4 CPU which you can try?
Maybe even put the case on the side with the side panel removed so the hot air can easly move out of the case.
The only real fix is replacing the motherboard. With the suggestions I made before you should be able to tell if it is really the motherboard.
See my posts about this board
MSI today announced that its full 300-series motherboard lineup, which includes A320, B350 and X370 chipset-based motherboards, now support AMD's latest Zen 3 CPUs in the Ryzen 5000 series. The full support will be backed into BIOSes developed under the latest AMD AGESA COMBO PI V2 1.2.0.7 BIOS...
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MSI today announced that its full 300-series motherboard lineup, which includes A320, B350 and X370 chipset-based motherboards, now support AMD's latest Zen 3 CPUs in the Ryzen 5000 series. The full support will be backed into BIOSes developed under the latest AMD AGESA COMBO PI V2 1.2.0.7 BIOS...
www.techpowerup.com
Specs below. My pc crashes and reboots when running demanding games at higher settings. Just today I had it crash on Like a Dragon: Ishin and Call of Duty MW2. Checking the event viewer gives no clear indication as far as I can tell, only an event ID 41 which reads "The system has rebooted...
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