Have you tried setting your speaker options to 5.1 in Windows? I know this should have nothing to do with S/PDIF out but a friend had a similar issue that was resolved by this. Also onboard Realtek audio, on an Asus board. We tried everything and all multichannel sound (DD or DTS) kept cutting out after 2 secs. He called me later to tell me he'd set the Windows speaker config to 5.1 (although he's only using digital out) and suddenly his S/PDIF passed DD and DTS ok.
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