Install these utils so you can gather historical data:

apt install atop sysstat

then:

service atop start
service sysstat start

My wild guess would be that every few days bots (ex: googlebots) try to re-index your site simultaneously or aggressively. And that causes the memory/cpu spikes you’re observing. It could also be malicious bots brute forcing your authentication pages. Review your access logs or analytics to try to understand what’s happening.

Cheers