This is a sign that file system permissions are not correct under /Data, or more specifically under /Data/Sites/[SiteID]/systemfiles
Things like SiteSettings and the SiteMap are cached in memory and the only way to clear the cache is to touch cache dependency files located there, but if file system permissions prevent creating or modifying those files then it fails to clear the cache. Eventually the app pool recycles and that will clear the cache. So when you edit site settings or add or delete pages it tries to touch those cache dependency files to clear the cache.
You can try by deleting all files from under that folder and if permissions are correct they should come back as needed.
Hope it helps,
Joe