Apple will return you your 49 bucks if they detect a hardware problem. Obviously, as you see on the Apple forums, it is the case.

Do not hesitate to ask for a midplane (the motherboard and related stuff): they can send you the replacement stuff (!! upt to two months of delay - they ran out of replacement parts) and you can do the operation: see manuals:
here

Do not hesitate to ask to discuss with the customer support; be quiet but firm and explains it is not normal for a computer (whatever its design is) of less than one year to have security shutdowns because of overheating

Good luck
Phil

Thanks for your help. The apple store will receive my imac and repair it. As above mentioned, I think is worth to pay the 279CHF for the Applecare plan.

Cheers,
P

Yes, my G5 is my 5-th Mac machine in my live.
Due to its design, the price of parts, I never imagine not to buy the AppleCare when I buy the machine.

I own a AMD 1.5Ghz PC. I never buy any extra warranty for this machine because I know that I can find at the corner of my street a DVD writer for less than 50 euros, the motherboard for less than 150 euros or the CPU for less than 200 euros.

Obviously, this is not the case with Apple parts and I do not believe the future MacTel machine will help.

AppleCare is not a option, nor a "must have": it is a part of the final Apple price.

Phil

problem solved
 

Thanks to all for your help!

I sent it to apple and they replaced the logic board without any charges.

First of all it doesn't sleep randomly anymore so I am able to work normally without interruptions and secondly I don't feel like working in front of a blender anymore.

Happily I paid the AppleCare protection plan, so hopefully I don't have any problems with the iMac at least until all macs are built with an Intel processor (end of 2007?).

P