As the Microsoft Article suggests it, have you looked at the Windows Event Viewer log and check what the message is ?
This would a good indicator about why the connection fails... it could be multiple sources of the problem, like Richard said, Firewall port blocked, network issues, authentication, etc..
You said this happens randomly, so I'd tend to suggest the problem is network related, since you say the server reboot fixes the problem.
Where is your MR 2012 Service installed ? is it on the same server where the SQL database is installed ? is it a dedicated apps server ? Can you check if the MR 2012 services are running effectively when you get the above error message ?
Lots of questions, but you really have to provide more details if you want us to help you.
