It has been 9 months since this post, but I usually do have the auto power set to turn the camera off after 8 minutes. These shots were occurring in rapid sequence after one another, well within the 8 minutes. Here is what Canon said about the warranty repair for the intermittent exposure problem: "it was found that the af assembly did not operate properly causing unevenness in the exposure. The af assembly was replaced." While I didn't have any auto focus problems, the camera has not had the problem occur again, so I guess that Canon fixed it.