Overheat?... or PSU overload
I've had the same prob several times over the years.
MY PC now runs with no side panels to aid ventilation and the PSU is 1000+Watts (Power causes increased temp output!)
I've improved from lots of 500GB HDDs to 2x 2TB HDDs with 2 optical drives to reduce power usage, so I agree with the above, it's probably not temp, but power usage causing the re-boots