ASRock K10N78FULLHD-HSLI HDMI audio problem

Hi all, I'm new on this forum. I have a problem, but I'm not sure that depends on the MB. I have an HTPC with a ASRock K10N78FULLHD-HSLI, with nVidia chipset, plugged in my LG LH4000 with an DVI-HDMI adapter. When I start a film, it works for about 5 minutes after this the audio suddenly interrupts. And it happends the same thing with all the players (VLC, mediaplayer, etc). With VLC, if I disable the sound trace and then I reactivate it, it works for another 5 minutes before it stops again. All the drivers are up to date and the HMDI device is set as default device. What's the problem? I'm going crazy with that!

ps: I use Windows Vista 32bit as OS.

Thanks