boot up the mac with a live CD from any linux distro and see if it behaves the same. If yes you have had a hardware failure somewhere and the machine should be repaired... if live CD is ok ssh into the ESXi and run "esxtop" to see which instance is consuming cpu power. From there track the issue possibly. If you are not able to, you might consider exporting the VMs out on an external disk (as a safety measure) and reinstall the ESXi.
Also although this is a home lab I would spend few quids on a UPS... it saved me lots of times
hi, thanks for your answer.
I booted the Mac from with its online diagnostic utility. after running the test I came back with no issues.
I'm now thinking its a software issue. I'm going to re-install ESXi on this host and see where that gets me.