I am having troubles with my MacBook Pro 15inch 2.4 Ghz as well. It is sometimes similar to problems described in posts above. My machine is around two years old.
Below are my notes and observations.
The previous week my MacBook Pro has displayed the following visual anomalies
1 very short flashes of black (fullscreen)
2 screen flickering (rapid flashes, like a stroboscope, stops and starts again after few seconds)
3 screen corruption: weird colors, clipped lines, image distortion (in Console.app, iPhoto, iTunes, etc.)
During these anomalies, the machine is very unresponsive (quite often I mistook it for an completely hung system). All user interaction (like clicking and keypresses) get executed several seconds and sometimes even minutes later.
But music continues to play and it is possible to make a screengrab (after waiting).
Turning the system on and off gives a kernel panic during boot most of the time. Booting using verbose mode (apple-v) did the trick most of the time, but sometimes stuff was so corrupted I had to do a boot with the Leopard install-dvd and do a repair disk and repair permissions.
Temperature measuring of cpu and gpu did not indicate an overheated system.
Problems are reproducible by doing a specific sort of graphic intensive task. Like clicking through your images in iPhoto, or turning on the Visualizer in iTunes. Running OpenGL games however did not result in corruption (still have to confirm this). I suspect applications using a framework like coreImage, but I still need to check this.
Also, I did run the hardware test:
The Apple Hardware Test reports the following error:
"4VDC/1/40000003:videocontroller"
During both the short and long tests.
When I turn off QuartzExtreme, using 'Quartz Debug' (in the Development tools) it seems to stop the corruption (but results in a almost unusable system due to slow graphic updates like moving windows or scrolling). Console.app still has graphical corruption during scrolling but will display correctly when one switches to another app.
As far as I know now it can be one of the following:
1 a defect graphic part (the nVidia 8600)
2 another hardware malfunction (motherboard?)
3 a bug in the recently released firmware upgrade for the MacBook Pro
4 some other software related issue (for example in the recent 10.5.7 update).
Any thoughts on this?