Well, I've looked into it a bit more, by comparing the Panoramio placemarks (which don't appear to have this problem) with this one, and I think I've narrowed it down:

My example is a webcam image, which gets refreshed every 5 minutes. So, the URL http://fjordcam.nedernorge.net/webcam320.jpg is, at all times, between 0 and 5 minutes old.

When Google Earth retrieves an image, which has a timestamp of at most 15 minutes ago (I determined with trial and error), it apparently treats it special (as a 'volatile' image or something). That means that it'll do a new request after 10 seconds.
This isn't so bad (albeit a bit fast), but, if it gets a 304 (Not Modified) response, it immediately re-checks the image, and keeps doing so, multiple times per second, until the image is actually changed, and it gets a 200 (OK) response.
At that time, it shows the updated image, and stops checking.
Strangely, it doesn't start checking after 10 seconds again, but this time it seems to wait about 2:30 minutes before the whole thing starts over again...

Obviously, this is a rather nasty bug. The webcam in question is now listed in the "Google Earth Community" layer in GE (but not (yet?) in the Geographic Web / Best of GEC layer), so it'll be viewed more often than before.

Now how to get the Google Earth developers to read (and fix!) this ...

- Michael