i have a PNY 6800 Ultra clocked @ 440 / 1200 whitout any problem.
Now if i clock the card at standard core e memory speed (400 - 1100) it just look up / freeze
I have contacted PNY and this is the answer :
"Dear costumers, the problem is the bios, the card is clocked at 400 mhz for core in 2D mode and at 425 in 3D, it's internal bios setting and the famous coolbit2 can't underclock your 3D mode, it still remaining a 425 also it's downclocked at 400 and the results are : 400 mhz 2D and 425 3D.
this settings are wrong and causes freezes and lockups.
I suggest you overclock your card at almost 430 mhz core speed in both 2D mode and 3D mode, in this case your result are : 430 / 430 in both mode and this settings are fine for all your games and application."
Pny tecnical stuff Europe.
P.S. I hace traducted the answer from italian to english, so sorry for bad language.
hope this help someone.