Add mine to the list!! Late 2006 C2D iMac. I can't remember when this started happening, maybe a year ago or so? I can't remember, but yes, the horizontal lines. Graphical artifacts / anomolies. Constant freezes; sometimes the machine can last for a few hours, sometimes only a few minutes.
I've had freezes where the screen just goes blank, and one where I get a blue screen with white/grey vertical pinstripes.
I haven't taken any remedial action at present bar the smcfancontrol, I haven't changed out RAM or done any sort of clean install (currently updated to 10.7.2).
I have run the AHT test on it, which picked nothing up but that was only one extended test.
I will look to replace the logic board myself (well, my mate who's good with this stuff will, I hope!) as I'd be gutted to see the computer scrapped, but this will be weighed off against whether I buy a new computer. I might leave it to see if anyone has any luck with legal action, not sure on how consumer law would relate to computers? It's generally accepted that a reasonable lifespan of a computer is three to four years ... five to six years??
If I do buy a new computer I'd be tempted to get another iMac, but, I would look around at PC/Windows alternatives.
I'd be interested to know how anyone get's on, personally I haven't got time to donate towards pursuing legal action, but if it were successful I'd certainly look to do that if there was a precedent set.
Luke