Re: 8 cores shutting down

A Mac Pro should only get up to about 70c or 160F, any higher there might be a problem. I do know the case can handle quite a lot of heat, the powermacs got up to 180F-190F. The new intels though are much cooler than the PPCs, and use less electricity.

If you need to you, you can always reset the SMC(another option I forgot about): http://docs.info.apple.com/article.html?artnum=304123

This normally does fix or help much, but in your case it might. Every time I have a computer that shuts down like this, Apple always says I need new parts.