that's weird, i've started having a similar problem. it's intermittant, so hard to nail down.
I've just rebuilt with an Abit NF7-M, XP2600+, 512MB DDR, and Radeon9600.
I used the Radeon in my old build (Gigabyte 6-VTXE) and didn't have the problem, so it's looking like a combination of vid card and/or mobo - or maybe we both have dodgy cables