I now have another problem with my HR24-500. Once every day, or two, the picture starts "vibrating" back and forth, left and right; don't know any other way to describe it. It is usually just part of the overall picture, sometimes there are several different "layers" that vibrate.
I turn off the DVR, turn it back on and the problem disappears for a day, or a couple of days. Then it reappears.
Symptoms fitting this description are common to both HR21-100 and HR21-700 going back a couple of years (my experience). Usually occurs while watching standard def Mpeg 2 channels, more likely during "live buffering" rather than recordings, but can happen occasionally in the case of recordings, as well.
It appears to me as though these machines aren't always properly tracking the interlacing, displaying the odd and even fields in the wrong order, causing the "vibrating" effect, especially pronounced during animated programming. It can happen very rarely, on an HD channel, but motion only seems slightly "stuttered" rather than "vibrating" in HD cases.
In any case, I usually just pause and unpause (or repeat button) the program and eventually with luck of the draw (50/50 odd/even) it straigtens out. In the rare case of HD, usually once is enough.