We have had the same problem on both a 622 and 722. I do not believe it is caused by high temperature. The 622 is sitting out in the open on top of a dresser and runs very cool. Our 722 is in a cabinet, but I have made special effort to ensure it is cool, and I have checked the temps. and they have always looked very good, based on other threads where that topic is discussed.
We are on our second 722. The first one had this problem really bad. It would happen several times a day. Sometimes skipping back and forward would make it go away, other times nothing would make it go away, even a reboot. Finally the audio dropped out on all HD channels completely, so I finally called Dish and they replaced it. The new 722 is much, much better; but we have noticed it a few times, and skipping forward and back fixed it quickly. The 622 still does it, but never as bad as our first 722.
The fact that we had one 722 that did it so bad, and a replacement showed such an improvement suggests to me a hardware problem. If so, given all the examples posted here, this could become a major quality problem for Dish. I hope not. Would be nice if it could be fixed with a software upgrade.