55C is a bit hot.... Even for an Athlon. But it should still be in operating limits. What I'm wondering, is your cooling for the athlon. If you used a sleeve type fan, it's possible that the fan seized during the evening, and cooked a bit.... Athlon's WILL burn up in about 5 seconds without a heatsink applied, and my only contribution would be that it's possible that with the fan in a stopped mode, the Athlon exceeded it's limits.

But the fact that you can run memtest points against that.

How far into the Linux boot do you get? any messages of relevance?

DIR works on the Floppy disk, and dies on the HD's, (ie, the floppy control is ok, but the IDE is gone?)

It's possible one of the bridges is faulty, and those can get hot, causing a marginal bridge to fail.

The other thing that may be happening is a bios problem. A re-flash may be in order.

Sure doesn't sound like a nice problem.