MID 2010 MBP 17" with Nvidia 330 Graphics
I also have the same problem - Run the Apple Hardware Test tool - result everything passed OK.
Reinstalled OS Snow Leopard - Still same problem will happend when external monitor is connected. Brought the Mac to the Apple Service Center - they tell me it is a disk failure - Changed with new disk - still problem remains same after new disk :-(......Cost me allready some for a NOFIX solution....
After sometime the Mac freezes up, and there are horisontal stripes on the screen - can still move the mousepointer - but everything else is not responding.... After reboot it will work Ok for a little while.
It seams like the problem is more likely to occure on a warm machine, and when a extranal monitor is attached.
I have now turned off the automatic graphics changing and have done the restting of PVram etc - and will see if the problem still remains - I hope the issue can be fixed soon.