We are actually still experimenting the issue, but randomly. Not always and not during the whole day. We are trying to identify if something is causing excessive CPU usage, as this results in the site becoming suddenly very slow. We have no idea why.
If you have access to origin logs at your hosting provider, maybe they can reveal something.
Also, (this might not be related, but) make sure Cloudflare IPs are not being rate limited or blocked.
You can follow this documentation to make sure they are allowed unmitigated access to the origin at all times: