I think what's happening is RAM deterioration. IIRC, my oldest and most used
(24/7 > 5 years) X600 started displaying the same kind of corruption. I
replaced it with another X600 off eBay for $6. Another I bought off eBay worked
OK at boot and on vttys, but corrupted like yours on starting X at modes higher
than 1280x1024 or so. With yours having only DVI and S-Video outputs it sounds
like yours came in a SFF Dell. I see a 5-lot of low profile X600 cards in a
current eBay ad for $19.95 delivered in the USA, and many other ads for X300
and X600 at low prices in US and abroad.