Why does my screen scramble after a short period of use?

My laptop has worked well since new but late last year the screen began to show unstable signs, gradually getting worse until it was impossible to use.

The screen developes a sort of coarse pixellated look and is difficult to read. After a short time as if reaching a certain temperature, these coarse pixel or really they're blocks of pixels worsen, making it useless.

Our local Apple repair centre had a look and diagnosed a fault on the motherboard which are either not available as a replacement part or expensive enough to make it an unattractive repair proposition. The tech said he'd never seen the like before. The model number is A1046 with a 1.25GHz processor.

Since then I've bought a M1138 1.67Ghz, which I believe to be the last Aluminium model (1440x960). Touchwood, flawless and a delight.