HD-PVR 2 audio intermittent?

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The HD-PVR 2 is recording over HDMI from Google GFTV200. The video records fine, but the audio intermittently fails.
In one case, 30 minutes into a show, the audio goes silent and remains off for the rest of the show. The 30 minute mark was precise to the second, and corresponded to the instant that another show started recording off the HD-HomeRun.
Audio returns on the HD-PVR after a random period of time.

While the audio is not working through the HD-PVR2, there is still audio at the HDMI output of the HD-PVR 2 if it is connected to an input on the receiver. It is just the recording/transfer of audio through SageTV that goes silent.

Where do I start debugging this?

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I believe I have resolved this issue by installing a small fan blowing on the HD PVR 2. It is installed on a shelf in a fairly open area that already had good natural ventilation, and the HD PVR 2 doesn't seem to get noticeably hot in operation, but the fact that the audio quit after a period of time recording got me thinking that could be thermal related.

Audio has recorded 100% from GFTV200 for a week. I have DD 5.1 audio recorded and played back through HD300, thanks to SHS help in the other thread.
I hope that is actually the root cause and not some other more difficult problem.