Ok folks, here is the deal. First, I'm a twenty year PC tech, and this problem is also common in PC laptops using the core duo. Per Intel:
"Intel Core Duo technology implements a new multiple-core thermal control algorithm. Both cores synchronize action requests and activity. A programmable policy can select DVS on both cores and linear control on each core either independently or as a locked operation. PROCHOT was also made available as an input, to allow thermal protection of platform components such as the voltage regulator (VR).
A finer grain overheat detection is performed by monitoring the thermal activity. If an extreme thermal condition occurs (fan malfunction, operation within a bag, etc.) the processor self thermal management may not be sufficient and the temperature may not drop below the threshold point. The internal control algorithm tracks the control behavior and further reduces power as needed. Continuous extreme conditions will eventually initiate an "Out Of Spec" thermal interrupt and register the warning in an internal status bit. The "Out Of Spec warning" signal is intended to initiate a managed system shutdown before a THERMTRIP shutdown occurs."
This "Out of Spec" signal comes when either one or both core exceed temp. Some have said that their Macbook wasn't hot at the time but "hot to the touch" or even using program isn't the issue of whether or not the shutdown occurs. For instance if the fan isn't operating correctly, or even if the processor runs below spec rated temp.
Confusing I know, but the Core Duo is loaded with such "flaws", as most "prototype" chips are. That's right the Core Duo is the 601 of Intel - a beta, which is why Core Duo 2 the true chip.
Sorry, but no matter how many times they replace the logic board or even give you a new one, this is a design flaw of the Core Duo.
See
http://www.geek.com/news/geeknews/2006Jan/bch20060123034350.htm for more info.
