The "Service Unavailable" error is an IIS error. I did some quick googling, and it seems to happen when the application pool stops (as you know, because your server works again when you restart it). The question is, why is your application pool stopping? From what I understand, the application pool can stop when there is a crash in a process that the IIS application pool is running, which sounds to me like it could be your problem.
Check out this article, it might help. A little long, but it might help you understand and diagnose your problem: http://blogs.msdn.com/david.wang/arc...ool_Crash.aspx
You might want to check out your system's event log for errors, to see if it reports when the IIS application pool might be crashing.
Which version of ColdFusion server are you running by the way? If it actually is ColdFusion that is causing the application pool to crash, it is probably because you have an old version. The latest version is 8.
Otherwise, you said every morning when you come in, you see that the service is unavailable. Do you perhaps have any scheduled tasks that run at night that could possibly be crashing out the IIS application pool? ColdFusion, asp.net, or otherwise?
Also, even though that article says it's usually not an operating system or IIS software issue that would cause an application pool crash, try getting the latest service pack and updates for your version of windows anyway. Also, if you can, try re-installing IIS itself.
Hope something in this helps, and good luck.