Unfortunately, it could be almost anything -- motherboard, RAM, PSU, CPU or chipset overheating, or software (e.g. buggy device driver, or even a malware infection).
Does the system seem to run stable otherwise, or have you seen unexplained crashes during other tasks as well?
First I'd run a memory diagnostic like Memtest86. Then make sure you've got the latest BIOS for the motherboard, and the latest chipset, NIC and video drivers installed.
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