For over 6 months my Mac Pro has been locking up.
 

Finally I had enough and opened it up. The inside was relatively dust free. Until I looked at the video card and the mesh was clogged solid with dust. I blew it out with a can of air and now....at 80 degrees in the attic, this Mac Pro has not locked up once. It's been a week. Used to lock up every 10 to 15 minutes.

So just FYI for those who may have been having problems with their Mac Pro locking up, check your video card.