hard shutdown is happening to prevent damage or PSU not being able to supply enough power for the whole rig to run.
typically, these run for 5-10 mins and then shutdown problems are thermal issues, since they don't present themselves during normal windows operation. Since you say your GPU and CPU are well below 85C it's clear that those are not the cause for shut downs. Check any other temps you can get to (hardware has thermal diodes) usually a mobo has a thermal diode around the CPU package or around ram. HDDs occasionally have a thermal diode too. PSU might have a dedicated, but for it to report that to mobo you would have to connect it by a separate wire.
if this is all new parts, open your computer up and check that everything is seated properly. feel the components if they are hot or not. etc. check for bulging capacitors on mobo (although those usually present much more severe
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