Performance Issues on WP site

Hi

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 ?

Thanks

Erik