503 errors For Only Logged in Users

Hi, I have been using wordpress for 4 months on the present hosting and never had a problem. But since 4 days, I am observing too many 503 errors. My hosting service have said everything is right fron their side.

The wierd thing is that it is only for logged in users. When logged in users get 503, at same time site is online for logged out users.

When we go offline for logged in user window, to get site online we have to open site in incognito, (as logged out user). And then reload the page in the normal window and it starts working. I have never seen site offline as a logged out user

My plugins have been same for 1 month and issue is week old. Still i believe the plugins that can cause issue are, Ultimate Member, Fast Velocity Minify, Autoptimize, Frontier Post.(becuase they are related to users)

Though i strongly believe that their is no plugin issue. My site is safe and virus free according to Sucuri. And as you can expect I have tried conflict test for plugins and themes.

The page I need help with: [log in to see the link]