I look at all the errors. If there’s cache coherency ones that suggests the CPU. If it’s only restart without clean shutdown that suggests more towards power. Another possibility is the A1000G’s more sensitive to the mains than whatever else you’ve been using, though if it’s a previous ATX desktop rather than something battery backed (laptop, tablet, phone) that strikes me as unlikely.
Wendell’s Intel’s got a problem - things to try is also a good starting point. Other commonly used tools for checking system stability include Memtest86+, OCCT, and Prime95. Like y-cruncher those are load oriented, though, rather than the light load failures that seem to be happening here. If the builder’s decent they already ran those or similar and the system passed, though it could be something with shipping.
Some other basic checks are looking at temperatures with HWiNFO64 to see if anything’s running unexpectedly hot, looking for motherboard warp from poor assembly or shipping, and checking cooler and socket torque. With the latter, if things are too loose or too tight LGA processors in general don’t make reliable electrical contact. Builder’s support might get unhappy if you dismount the cooler, though.