although normal temperatures were a bit on the high side originally at 80+ degrees.
!!!! The 6600 runs quite
cooler than that , the core should be under 40 degrees . I think its a simple heatsink reseating issue. Check out the fan of the card and see if its running ? If yes then check that the heat sink touches the core properly . I am sure it will be alright once the card goes around 40 degrees ..
PS : We had this issue on a friends 9800 pro , the heatsink got out of its place and it started showing artifacts at high temps. Once we reseated the heatsink with a good thermal compound , the card began working perfectly. And oh yeah aren't black artifacts an indication of core ?..... Memory instability causes white snowing .... I think ...