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.
If the world isn't making sense to you, you're either drinking too much or not drinking enough.