I've sent my 820 off today, as instructed by Garmin, for them to replace it under warranty due to the repeated random / unreliable behaviour when trying to follow a route / course.
I really don't see how this can be anything other than a software problem, particularly as others are suffering the same problem.
It is possible, I guess, that there was a common hardware problem with early units and that it will be solved in the replacement unit. But I'm not confident.
Can anyone report back that they've had a replacement that has solved their problems?