Thanks for the suggestions.
Did the Memtest in my old system today with the same RAM. This time all modules passed without a single error, so bad RAM modules is definitely ruled out.
Higher RAM voltages don't seem to help. I tried up to 2.20 volt, not sure how save higher values are.
I contacted Asus with a few specific questions, should get a reply within 48 hours. Depending on their reply i'll probably have the board replaced and hope that fixes everything.
Also posted this issue on Neowin, there are a few more suggestion there but those didn't help either:
http://www.neowin.net/forum/index.php?showtopic=650295
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