Cloudflare Wordpress HTTPS blog - randomly goes Internal Error 500 - debugging the issue

I am literate when it comes to coding, Wordpress blogs and hosting sites and stuff like that but I have no idea whats going on. Im trying to debug an issue.

Facts:

1. My Wordpress Cloudflare Site goes internal error 500 randomly for 10 minutes then starts working again. Then it will work for from 1 to 15 hours till it gets error 500 for 10min again.
   
2. It started happening long ago so i have troubles locating the issue.
3. My wordpress uses SSL FULL and other enhancements from cloudflare
4. Other domains on this hosting (same server) work fine. This is also a very fast hosting that i used for many years without issues.
5. Theme Schema by themeforest with custom php edits. However ive recently OPTIMIZED these custom php edits so they shouldnt cause this.
6. Plugins I use (its probably one of the plugins thats culprit):
   1. https://wordpress.org/plugins/ad-inserter/
   2. https://wordpress.org/plugins/buttons-to-edit-next-previous-post/
   3. Contact Form 7
   4. https://wordpress.org/plugins/contextual-related-posts/
   5. https://wordpress.org/plugins/disqus-conditional-load/
   6. https://wordpress.org/plugins/disqus-comment-system/
   7. https://wordpress.org/plugins/easy-table-of-contents/
   8. https://wordpress.org/plugins/enlighter/
   9. https://wordpress.org/plugins/imagify/
   10. https://wordpress.org/plugins/fast-indexing-api/
   11. https://linkwhisper.com/
   12. MyThemeShop Theme & Plugin Updater
   13. https://wordpress.org/plugins/host-webfonts-local/
   14. https://wordpress.org/plugins/post-order-by-category/
   15. https://wordpress.org/plugins/seo-by-rank-math/
   16. https://wordpress.org/plugins/responsify-wp/
   17. https://wordpress.org/plugins/mailin/
   18. Stop Generating Image Sizes
   19. Super Socializer
   20. UpdraftPlus - Backup/Restore (PREMIUM)
   21. WordPress Popular Posts
   22. WP Post Navigation
   23. WP Rocket
   I thinkits one of the plugins but i dont know which one. I cant jsut turn them off one by one because i get this error randomly once per 14 hours or something.

Pingdom monitoring:
https://i.imgur.com/yyy1BHn.png


(but im not sure if its related to ssl)

I appreciate any help!