That’s not a theme issue so it is beyond the scope of theme support. If you need help with basic questions like that you may want to consider hiring a professional web developer to help you?
The easiest thing to do is to deactivate all plugins, all at the same time, and then check if the problem is still there or not. If the problem has gone away, reactivate your plugins one at a time (individually, not incrementally) until you find the plugin that is causing the problem.
But your next step should be to contact your web host. Ask them specifically if they are using Memcached or any similar caching tool that is based on the memcached code. Ask them to disable all caching for your site.