Finder Using Nearly 100% CPU

I think I found my problem—one or more of the files on my desktop.

After moving the various clutter off my desktop, the Finder stopped using so much CPU.

I suspect that it was either calculating folder sizes (as Benjamin Schollnick mentioned) or maybe trying to create an icon preview.

Check Finder --> View --> View Options

If you are viewing a folder, especially one that has subfolders, and you have Calculate Folder Size turned on, the finder will have to go through and "size" all the folders...

I noticed that my Mac Pro will take about a minute or two minutes of nearly 100% usage when I initially turn that on, or if I start viewing the disk from the root.

Try turning that off and see if that resolves your issue... It's a work around, but it'll help isolate the issue.

An easier way to restart Finder is to option-click on its icon on the Dock and choose "relaunch".

This works for a while for me, but the reason behind Finder using so much resources is tick on the box "calculate all sizes" as you said. This takes up much energy especially after major reorganizing/cleaning up the harddrive.