Also had the same issues. Restarting the Canon service solved the problem. Thanks for your help.
What I found was that when I added a new printer, in this case for a Datamax (using Nicelabel drivers) the service stopped. It was even stopped when I went to share the printer!
Very annoying but at least I know what to look out for if it happens again!
After I had setup the new printers and re-setarted the service it has remained stable.
We are using the PCL6 driver.