Nasty, I read the ratings when the card hit the market. It's a heavy duty card, but suggested ratings and real world usage are two totally different things. As a hardware mfg I can recommend you to get an 800W PSU if I think the world is littered with shitty 700W PSU's. Recommending a larger than necessary PSU discourages those who run crappy non-80+ PSU's from buying your part, burning up their gear, and coming around looking for warranty services.
Real world usage is the amount of power the part actually draws off a kill-a-watt. I'm getting 400W from the benchmarks that I posted. Been reading Anand for a very long time, the benches are trustworthy. An average system coupled with a single GTX580 should pull around 400W. Any PSU that can keep at least that on the rail, coupled with a higher peak, and the recommended amperage will work with the gear.
We're talking about a PSU that pushes 69A on three rails, rated for 550W. It's the bare minimum I'd go with, but there's no reason it shouldn't work...unless of course the PSU is failing. I'd spin 2 rails to that card. My hunch is that it's only sitting on 1 right now, and that won't cut it.
Case in point, my Radeon 5830 recommends a 500W PSU...http://www.amd.com/us/products/desktop/ ... iew.aspx#3
Been running it off a Silverstone single 42A rail 450W for over a year now. Whole system overclocked to the gills and liquid cooled draws less than 300W under full load.
Also consider the fact that the video card isn't even under
load when the OP is having problems.