Overheating Confusion, Smart people Needed!

[rockdogz]

This sounds a lot like what happened to me recently, and it was my water pump. The pump only had about 2000 miles on it, so the bearing was fine and there was no free play in it. What happened was that the impeller separated from the shaft... see the picture here. Needless to say it wasn't moving any water around the cooling system.
It was a Stewart pump too, so I guess you never know when something freaky like this will happen. They did rebuild it for me so that's good.