It means you either have the extra power cables incorrectly put or your power supply dies.

Get more power. I think 600+ is the way to go nowdays, I have 850W dual heatpipe myself. I had similiar issues when I first time overclocked my GPU. Apparently the power cables weren't on perfectly.

EDIT: Another possibility is also overheating. Try benchmarking your GPU. Get monitors and make it do something. If the heat goes well over 80 celsius easily, it's overheating.