Audio dropout via optical output to receiver

I have a Auzentech Prelude 7.1 soundcard that outputs Dolby 5.1 and DTS. I recently bought a Pioneer Elite 21TXH receiver. I have my video card (GTS 250) going to my receiver via dvi to hdmi converter. For sound I use the optical output.


My software is setup to output Dolby Digital, which it does, and my receiver recognizes it without a problem.


The problem is when I play .mkvs that I have. Occasionally, only happened when watching .mkv's in MPC-HC, usually about 20 minutes in, the receiver will drop the audio and display 'stereo' like it's not receiving a feed. Once I exit Media Play Classic Home Cinema, it goes back to Dolby Digital. It seems as if MPC is throwing the receiver off (handshake issue perhaps?). Sometimes it will just kind of crackle. A close and reopen of MPC gets it back up and running.


I am running Windows 7 64-bit. I have tried running split screen and just single screen. I usually play the video in 24hz. I have tried VLC and Windows media player, but both are awful when playing in 24hz. I tried playing it in 60hz on windows media player but it still happened.


I tried the optical cable with my PS3 and it seemed fine (only tested it for about 10 minutes). I don't think it's the optical cable.


I have played music through the receiver from the PC without a hitch...the only dropouts I experience are when viewing a 720p .mkv.


I'm going to send music all night to the receiver to see if I wake up to it saying 'stereo.'


Any suggestions?


Thank you.