Ubuntu Server 17.10 was randomly freezing / locking up after periods of time, even if left idle.  The solution for this VM was to change the type of network adapter assigned to the guest. By default, ESXi 6.5 will assign an adapter type of VMXNET 3 for a new guest VM. Switching it to E1000 in the VM settings seems to have resolved the problem.

If running Ubuntu Server you may need to perform the following steps to enable the new adapter after changing the adapter type for the VM:

First get a list of all adapters currently detected. Make note of the interface name (e.g., abc12)

Then enable the interface using the interface name recorded above

Then enable DHCP client for the interface

You can now check if the system has obtained an IP address by using the following command:

To make this change permanent so that it persists with every reboot follow these steps:

Edit the following file:
/etc/network/interfaces

Then add the following lines. They may already be present referencing the old adapter name so you can simply update the name of the adapter.