Remember this post?
After long time and numerous investigations (thx to my colleague taking time to analyze the hang dumps!) we finally found out:
1. The problem is encapsulated in Samsung SATA HDD used as secondary drive: all the intensive file system operations led to freeze state of the box.
2. The OptiFlex 740 is known for problems with Samsung HDDs (look at OptiFlex recommended downloads list - navigate to download site and search for OptiFlex 740). Even W2K3 can hang using some Samsung HDDs (fortunately, my box had no problems with Samsung HDDs under W2K3).
3. Dell, nVidia and AMD do not provide actually complete list of drivers for Windows Server 2008 R2: for the moment of this post there are no installable drivers for the chipset (nVidia 6100/6150) and graphis adapter (in my configuration - 256 MB ATI dual monitor card).
We run stress test to confirm the problem is solved.
By the way: the "bad" HDD was put into external USB SATA<-->USB adapter and does work without problems as external disk now...