Menus do not load on some pages
Sometimes it is worth visiting /civicrm/admin/menu?reset=1 and right clicking on any menu item, and then click Edit and Save so that you then get the message


  Click here (/civicrm/admin/menu?reset=1) to reload the page and see your changes in the menu bar above.


Otherwise you can just visit that url directly. This may not help with your situation since it seems to be less consistent. But it may help others looking around for such advice