Many 3D cards require a minimum voltage to be stable, but when "idle", they are basically asleep so everything should be ok.
I have strong reasons to believe this analysis over any other one, that never lead to a solution.
I have the problem about once a day in average (but sometimes never in a whole week) since ages and have tried pretty everything without luck...
Her my observations:
I NEVER had the problem when the GPU was under load, NEVER when e.g. PowerDirector is converting HD software and asks raw power from the GPU, so it's NOT overheating, for sure!
I NEVER have seen the problem neither when the computer was full idle.
But what I am sure is that to 90% it happens when the computer was idle for at least one or two minutes immediately after I begun to work again (frequently upon starty an eMail reply with Thunderbird).
So all that confirms my strong beliving that it is an undervoltage problem.
I came to the idea to combine a potential solution with some public wellfare:
I will install BOINC http://boinc.berkeley.edu/download.php and let my GPU work for the science e.g.
Rosetta@home with 20-30 % of its computing power, so it will not be idle. ;-)
Logically the undervoltage problem should disappear.
Did someone try that?
You may do it as well and return your experience...
Looking forward to hearing from you