I am also having the issue described here
. I am the James Clayton who's posts from the Apple support forum feature in the discussion on the topic manchaca pasted into his report. That discussion can be found in full at https://discussions.apple.com/thread/2570597
, though Apple are allegedly now deleting posts from the thread.
Obviously Apple are an American company, but the issue appears to be widespread, affecting all models featuring the X1600 chipset sold worldwide. I myself am in the UK. The issue also dates back some time. My machine, purchased in 2007, has been broken for over a year, making it less than four years old at the time the fault developed. There do appear to have also been reports of Apple replacing the logic boards of machines that developed the fault within the warrantee period, so they should be aware that it's a common problem.
The older these systems get, the less likely it seems that Apple will acknowledge the problem, particularly as they've now stopped even shipping OS updates for X1600 based models. There are various solutions proposed by ingenious individuals on the Apple forum, but they are all too high risk to try on my only machine, or significantly reduce functionality. I have the graphics drivers disabled at the moment, which keeps the machine stable, but renders it practically unusable. In contrast, the far older PowerBook G4 I'm using as a back up is slow, but functions perfectly.
As long a I remain unable to afford a replacement machine, I remain interested in any action that might be taken to encourage Apple to acknowledge the fault, and offer a more reasonable solution than the 400 they charge for a replacement logic board. If my data were not tied into their systems, I could buy a decent brand new machine from another manufacturer for that.