Lol guys i have the same thing with my 260gtx I just get different color of pixel on my screen nearly everytime i load up my computer, have you found the way to fix that yet?

Yeah that's unusal they probably know 1 core gets hotter so they backed off the voltage.  Since we know on the 400 series 1.000v at 320bit can maintain 608 with low temps.  However say your voltage regulators were regularly reaching temps beyond 80C.  We may then want to consider a more ideal memory clock, vreg voltage to reduce vreg heat, and a more ideal memory clock for stability. Since vga ram in excess of 80C along with the gpu core can become unstable.  So I'd say extract your bios file take into nibitor and take a look at fermi voltage first.  What you can try is what should be a very nice low temperature stable P15 voltage of 0.950 on the left pane and 0.950 on the right.  According to fermi specs it can maintain stability of 810mhz at 0.950v but since the 384-bit per gpu talks some serious business.  You must add voltage sadly, which may draw too much power and droop your 12.000v rail into the 11.800v range and that's a clear sign of requiring a multi-rail power supply with say 4 12v rails at 15A each.  Anyway a card for good comparison is the gtx 470 fermi since it's the same basic architecture minus various features but same clock stepping to voltage requirement.
 
Anyway the 470 had 320bit ram which required it to use 1.000v at 607mhz core, 1672mhz ram.  Then you'll find that it can maintain stability at 0.9750v for 651mhz ie 1302 shader, and a nice 1800mhz memory clock.  I found even 0.950v was enough to maintain 651mhz all day.  So if applied to the gtx 590 could be very stable. 
 
Here's a screenshot of a nicely balanced bios configuration for the gtx 590.  Save as a new file 590bios.rom open in nvflash on ur dos flash drive.  Nvflash 590bios.rom -5 -6   ...  should all be dialed in press y continue wait, then it may ask again press y for gpu #2.  Once done crtl-alt-del restart back into windows now your core clocks on both gpu's will be at 0.9505v at 651mhz core clock.  Once we get more familiar with what the card is and isn't happy with.  We can then proceed to notch the volts up no more 1.000v, more like 0.975v which may allow 750mhz don't quote me on that though.
 

GTX590 are fixed voltage . You have to change the bios of the video card witch voids all warranty .but it does fix most all problems with the cards