Try using Asus Probe II. That way you can monitor the temperatures and voltages, and one of those are causing reboots, you would see which one.
All except VGA. Temp for VGA can be monitored in advanced settings in display properties. I have NVIDIA VGA and GeForce tab in that settings. On that tab I can see current VGA temp.
I bet it is VGA overheating.
Anyway, if the heatsink isn't properly seated, it won't pick up heat from the chip below, thus it will be cold to the touch, while the chip is overheating.
It has to be TIGHT, and thermal paste layer should be VERY thin.
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Posting Virtuoso
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since May 2006