Hello Marius,
I handled similar issue sometime back and possibly there are few VMs on the ESXi host randomly spiking up either due to backups or application asking more cpu/mem.. which you can see on the performance charts.
You might have to check the power state settings on the ESXi host which can impact the RDY % and lead to CPU spike up. Disable the power management settings on the BIOS (requires downtime) and monitor the environment
Thanks,
MS