I aslo got same issue but don't know how to fix it.
I have try to change from server name to ipaddress, the monitor will be up again, but next day the problem will occur again.
I have a few questions:
1. Which Communication type are you using (RPC or WMI)?
2. Are these local or mapped drives?
3. When you get the error, are you able to hit the Force Test button in the monitor, and get a reply?