Unfortunately, it is not possible to do the tests above without the fear that it might cause some issues on your live website. I strongly suggest that you contact your hosting service provider and ask them to have a complete backup of the site or move your website to a staging server so that your live site will be safe.
Also there are points in my colleagues steps that are not tech related and you do need to follow up:
1.Test for a plugin conflict. You can do this by deactivating all third-party plugins, and see if the problem remains. If it's fixed, you'll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
2. Contact your host to increase your allocated memory. It should be 512. I installed the WP Server Info plugin which you can check from WordPress Dashboard > WP ServerInfo and it shows the memory limit for your server is 256.