I have done a test install with Windows 7 Ultimate, and this bug dosen't show up. Watching recordings made with RC3 and RC2 (Still have some from before the change to rc3) works and the audio don't loose sync.
Installed RC6 (not completely clean but since I had some issues, I uninstalled MePo + TvServer and installed them again) but I still have this problem.
The only time I have seen this happen is when I set my screen resolution to 1920 x 1080. Clearly my graphics card couldn't cope. As soon as I lowered the resolution, it was fine again.
This may not be relevant to everyone (or indeed anyone) but I thought I should mention it as a possible issue.
Mark,
You may be completely correct about the video card. My card is marginal for HD, but something else has changed because from the time I started using MediaPortal until 1.1 RC1 or so I never had a problem even with HD content. The shows I record are the same, but the MP behavior has changed over time. I can't pinpoint the exact SVN, but it was around RC1.