Did you solve this? I've got exactly the same issue, it started about a week ago. I haven't done any changes to the system for a few weeks so it shouldn't be related to something like that. I couldn't find anything in the eventlogs in WHS pointing to any
kind of issue so I'm not sure what to think at this point.
When I reboot it can take anything from 1 hour to 3 days before it stops responding and it's quite annoying.