Going to a page that has a few images on it. IF you use that page a bit, after a while (maybe 30 minutes or so) the page no longer caches the images, each time the page is accessed that images are reloaded rather than coming from cache. This produces page "flashing" and slow loading of images. If the browser is closed and reopen, it will go back to working as expected. This has occurred are various pages and is not site specific. It also happens when using either Safari 4 or Chromium (mac) so it appears to be some underlaying problem that both browsers have in common. Thus, it would seem it could be webkit itself that is causing the issue.
I see you said that this happens on various sites, but could you please provide a definitive algorithm to reproduce this anyway?