Are you using high-performance memory by any chance, like Corsair XMS, OCZ,
Mushkin, esp. PC3200?

I'm using a Kingston pc 2700 and generic PC2700. I don't think this is a
memory issue.

Also, is this perhaps a "mobile" Athlon processor? Is the PSU a quality
unit? What model of mobo?

No, regular athlon & Shuttle AN35 mobo

Grativo