Occasionally my ZHA devices become unavailable and they come back after a HA restart.
Can I create an automation that restarts HA when ZHA becomes unavailable ?
For sure - but that’s just a work’a’round 
Prefert is probably to fix the problems which let ZHA became unavailable in the first place 
very true, but I don’t have a clue why this happens, as I wrote, occasionally.
This are indeed the worst problems to debug. Not sure if it is worth for you set up a some logs which will get written to a file so that you have something to debug the moment things go south again 
Back to my question… 
How do I create that automation ?
Have your connected the Zigbee Cordinator USB Stick to the server with an extension cable? It may get interference from USB 3 and Bluetooth when connected directly into the USB Port.
I have the Yellow Box with integrated Zigbee.
The service to call is homeassistant.reload_config_entry
Thanks !!! What trigger would I use to identify that ZHA is not available ?