60-70C shudnt cause a shutdown but as u said u cancelled the movie so i guess it goes above 70 and then shuts down.
You said normally temps were 40-50C.is that at Idle or at Load(i.e with a program like DVD Shrink).
I still think that the heatsink isnt making proper contact with the CPU.
Since u have Core 2 Duo ,try
Coretempand post the temps here .
Are you overclocking your CPU ?
hope this helps.