Long time no blog, been really busy unfortunately depending which way you look at it.
Recently we have noticed an increase in VMs failing to reboot correctly on soft-boot.
It looks more and more like we have run into a bug between Microsoft and VMWare. Whereby the TSC is not being reset correctly upon reboot, if the VM has been up for more than ~60 or ~100 Days, depending upon the configuration.
VMWare tells me that this is by design of VMWare Hardware v10, and is to bring the vHardware inline with Microsoft Server Virtualization Validation Program, and keeping a better clock sync within a VM.
From here, if you are experiencing the issue, you have a couple of options,
1A) If you are running ESXi 5.5, update to 5.5 U3 when it becomes GA
1B) If you are running ESXi 6.0, update to 6.0 U1 when it becomes GA
3) Continue to hard-reset any affected VMs as they arise.