A few angles:

• Power supply sagging under load or bad DC jack (laptops).
• Thermal shutdowns from poor cooling or dried paste.
• Faulty RAM or unstable XMP/overclocks.
• Driver or kernel crashes; check Event Viewer for BugCheck entries.
• Short circuits from a loose standoff or debris inside the case.

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What it might be (likely causes)

• Power delivery problems
  A PSU that’s aging, underrated, or tripping OCP (over‑current protection) can reboot the system under GPU/CPU spikes. On laptops, a loose/burned DC jack or a failing charger can cause momentary brownouts.