I hope you are well today and thank you for your question.
This can be due to plugin conflict on your site so please try temporary deactivating all plugins and see whether everything works fine and then enable the plugins one by one to see which plugin is conflicting if any.
You can also use the following plugin to find out the plugin which is taking too much of your site resources.
If it is working fine after deactivating plugins then it seems the plugin issue so please find out which plugin is causing it as described above.
Also try resetting your site .htaccess file as described on the below page because some plugins might have added code in it.