That's a very very very old video card. But even the cheesiest 550W supply I've seen delivers 450 Watts with ease. I'll write it's more likely a heat issue or some part you recycled has begun to fail.
Bob
Soyo p4x400, via chipset, 512 corsair pc3200 running at 333, (personally I dont think via is true 400 mhz but it loops either way) ATI radeon 9500 pro, cheap 550 watt powers supply. (I cant remember the brand but the 250 that was in my k62 was significantly heavier)
I've tried drivers for both the video card and the motherboard, bios settings and rage 3d tweaker but nothing seems to affect it. It will loop on the destop in a game or when the screen is dark and the hd has spun down.

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