i figured out why my pc was freezing.
the problem was
drumroll
MSI Afterburner.
if it's open and a custom fan curve is applied then my pc crashes after a while.
if it's not open (i uninstalled it) then everything is fine.
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@DrMacintosh i could live with 5 minute shutdown time. the symptoms here were a bit more severe.
the symptom was that after some gaming the pc would lock up, like you can hear it in the audio that it's frozen, the displays would go black and the gpu fan would spin up to 100% and stay there until you force rebooted the pc.
that's a little more annoying than a long shutdown.