Below is a CMOS battery that I bought for a friend's build to help troubleshoot a similar problem. In his case, it was the solution to rebooting in the midst of gaming, although I admit that this is a rare cause.
Duracell - 2032 3V Lithium Coin Battery - with Bitter Coating - 1 Count
$1.95 (select One-time purchase on listing page at time of purchase)
https://www.amazon.com/gp/product/B07GN8ZYZR/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Question: "450 w psu maxipower": Is maxipower the brand, or you meant to write 450 watt max power? If so, it's important that we know both the manufacturer
and model of your power supply. If you're not certain after visually looking inside your case, then please take a photo of the label on the side of it, upload the picture to imgur and share the link here.
Question: Just to be clear, your computer is actually shutting down all power, and not simply rebooting? And you're certain the screen image isn't just going black? When this issue happens, the lights on the case got off, and all fans inside your build stop spinning at this point in time?
I don't yet know what's causing your problem, although your answers to the above questions will be helpful. Power, software or heat are the usual culprits. With that said, CPU thermal throttling results in reboots, not shutdowns, in my experience. Either software or the power supply can be responsible for shutting down the PC, but I would think the power supply would be the much more likely culprit. My guess is that the power supply is the cause, and you also happen to have a dead CMOS battery, that's resulting in the cpu message. Even if the CMOS battery isn't the cause, I still would recommend changing it, as it only costs $2. As for the power supply, do you have one that you can borrow from a friend to further troubleshoot this problem?