I am getting some occasional extreme stuttering of both the video and audio from my HR21-700.
But if I rewind anywhere from 1 second to back to the beginning it is not there when I get to it?

It has been talked about also in the HR20/HR21 dedicated thread's. I don't think there is an answer but it has been talked out. I know me for one have experienced it during playback.

Not true, let me clarify. Sometimes I have video pause with uninterrupted audio. And this weekend on my free preview of movie channels I had numerous occasions where the audio was skipping like a bad record player. Press the 6 second rewind and things work wonderfully the 2nd time around.