Since the problem is not easily duplicated, it could be response times of your hosting place. It could also be some external javascript code required by theme or plugin not loading properly due to timeouts.
You could try to see what element is causing the problem by using the Network tab of the Code Inspector (F12) in your browser. That might help with narrowing down possible issues. It may take a bit of time to find, since the problem is random.
If it is some JS code not being loaded from an external location, then the theme/plugin should be changed to get their code from a CDN, which should provide the code faster. That requires changes to the theme or plugin, so you will need to contact their support. Don’t change theme code unless you use a Child Theme.