Performance Issues on WP site

we have a WP site.
It's a simple site - no WPML or WOO - just posts, about 20K posts.
We have wp-rocekt and cloudflare, not too much traffics - but the site crushes every X hours. After crush we need to re-save the PERMALINKS ...
Not 'too much' traffic - less than 10 at a time (according to analytics).
Moved the site to a dedicated VPS (single site on this machine)

After trying to understand why, debugging any doing anything we can think of- We would like to ask for your help to find the problem. Can you assist ?