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Well, I installed Xubuntu from a live CD. The sluggishness really doesn't become noticeable until the CPU is under load though. That didn't really happen until after it was installed. When booting from the CD I did see errors similar to:

kernel: [5585143.108121] EDAC MC1: 1 CE memory scrubbing error on CPU_SrcID#1_Channel#0_DIMM#0...

As I understand it this usually indicates a correctable memory error.

Windows also logged correctable hardware errors. It's just odd that problem goes away after putting the computer to sleep and waking it up and a reboot (soft or hard) brings the problem back. At least that works in Windows. I discovered that completely by accident so I didn't try it in Linux to see if it fixed the issue there as well.

I've never heard of anything like it... Is the computer still under warranty?

No, it is a many years old computer long out of warranty. I'm really just trying to learn something and satisfy my own curiosity.

I hope you will be able to find the answer. Things like that are a good challenge :) ...