I was researching this problem and discovered this review of an Asus FX5900 Ultra 256 that apparently is the only video card that is not manufactured by Nvidia completely. Check it out: http://firingsquad.gamers.com/hardwa..._ultra_review/
I didn't know this before, I thought each manufacturer made their own PCB's and just used the Nvidia chips...turns out I was wrong.
I still believe it is a power related issue whether from the main power supply feeding the auxiliary power connector on the video card fluctating due to poor quality power supplies or probably more likely the voltage regulation circuitry on the video cards is not up to the task of providing constant power to the power hungry chips.
I sure hope they sort this out soon so I can stay with Nvidia, I don't really want to get the ATI 9800 Pro 256M but I will if I have to to avoid these type of problems.