Windows 7: Sound and picture cut out over hdmi

Wonder if anyone can offer some suggestions to fix my problem: Acer pc connected to pioneer amp connected to pioneer plasma via hdmi. Recently the picture goes black for a few seconds and the audio cuts out.

I have now run the hdmi to the plasma from the pc and an optical (spdif) to amp/receiver via a cheap external usb soundcard from the pc. This solves the problem but I would rather have the one hdmi as its simplier and presumeably slightly more direct for sound.

I also have tried connecting the hdmi as it is as the moment (to the plasma) and then selecting to output the audio from the tv to the amp via optical (which is already there for cable tv). This resulted in the audio still cutting out but the picture being fine.

The audio/picture cutting out is completely random and can happen every minute or not for days so its very annoying and hard to find the cause. I do find that I can almost make it cut out by playing the media player and opening youtube (ie 2 things playing at once).

I've noticed in device manager that I have realtek audio device and nvidia high definition audio device plus obviously the usb soundcard I have added. I'm not fantastic on the computer but can carry out straightforward tasks. I've checked windows updates for the drivers which says they're up to date. Why do I have nvidia and realtek? Which would be preferable? Could this fix the problem? Any suggestions appreciated.