Your video card is not getting enough power, and is artifacting.
I would think that if the card isn't getting enough juice it would downclock. Anyways I would also think that PSU is more than adequate, if it's not gone bad.
Would you do me a favor and have a look in your eventviewer. Control panel -> system and security -> administrative tools.
Open the eventviewer and see if any error occur at the time when your system locks up. Check Windows logs -> system. If there is something of note there, please post the eventid.
I'm trying myself to troubleshoot something similar and so far I'm inclined to think it is a windows 7 problem.
What power settings are you running. Try in control panel to deaktivate the power saving option under advanced on your pci-e socket.
Hope it helps.
Edited by hamluis, 13 March 2011 - 07:27 PM.
Split from different topic.