What new hardware have you installed recently? If this happened just after a hardware upgrade, reasons we need which devices where upgraded.
As we then do not have as much to go on.
Overheating is definitely one cause to make the PC shut down if temps exceed cutoff level for CPU.
PC needs adequate ventilation, can be due to the area it is in, the ambient room temps, if it is closed in a cabinet, dust on vent and fans and also a bad CPU fan.
How are you able to monitor temps, saying they seem normal. Try HWMonitor, post a screen shot http://www.cpuid.com.../hwmonitor.html
Check the fan speeds with Speedfan http://www.almico.co...ad.php speedfan
Check in Event Viewer for Errors marked in red. Not sure operating system look here on how to do it. http://windows.micro...r#1TC=windows-7
Other causes could be bad RAM, bad Power supply, improper voltage. hwmonitor I gave the link to can check voltage on Power Supply.