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Thanks a lot for the offers and Scott for already having sent me 
detailed data from his new MBP.
I've been looking into it (=> some more info in the Blog) and even 
did some AppleTV hacking to see what has changed in hardware.
Result for now is that I've ordered a new MBP and will do further 
investigation as soon as it arrives and see what can be done to solve 
the problem.
Best regards, 
Felix