Web-site loading too slow. Sever request time over 60s for a 200 KB page.

Dear wordpress and web gurus, please help!

The problem:
I am having a performance-related issue with my website. The issue comes and goes by itself. A few times a day, sometimes for upto a few hours my website starts to load very slowly. My home page is only 200 KB and it takes 40-60 seconds to load. I’ve run a few load tests which showed me that it takes that long for a request to be sent to the server. In that time nothing happens, the web page remains blank and nothing is loading. At the end of that 60 second time period the website loads instantly (in less than 2 seconds) as it should. Also, when this issue occurs I am not able to access my Admin Panel (error I get: The server encountered an internal error or misconfiguration and was unable to complete your request.). After a while the web-site gets back to normal again.

What I’ve tried to do:
I’ve tried to disable all plugins. This didn’t help.
I’ve changed the permalinks back to default. No impact
I’ve spoken to my hosting provider who said that they didn’t have any issues with their server or db. I also have another domain and site on the same hosting account (with no database) which works fine.

My website:
https://www.stockbenefit.com.

Please help.