However, the VNC service occasionally crashes, and locks my session. It also refuses any more VNC logins. This is particularly frustrating when I am off site! Apple has analyzed this and has no explanation or fix.

Here solution howto fix it: If SSH access is enabled on the server, you can connect to the server via an SSH client  and run this command:

sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -restart -agent

You must log in with admin rights, and must enter the admin password again to run the command. It can take a minute to run. After it finishes you’ll able to login again via VNC.

You can play with remotely once you kickstart ARD
http://support.apple.com/kb/HT2370?viewlocale=en_US

Restart VNC service REMOTELY! is a post from: My MacOS X