I don't think the CPU is the problem. In my experience it might be the VIAboard and especially the memory itself. At my work we have had some troubles with the EK series. It looks like the SP series are form before that.
From the 25 devices we build, the ones with a DDR333/512MB kingston memory module were the most stable. DDR400 or 1GB in most cases worked too. Dane, Corsair, Geil, and another brand i cannot remember all failed sooner or later with exactly what you describe, a complete system freeze. Even a kernel debugger didn't give a hint.