RPC SErver is Unavailable

Now I only know RPC is Unavailable because the SolarWinds WMI Monitor (Points to our DC with Exchange Windows 2008 NOT R2) is telling me so, I have tried most of the check in the services on my XP PC and they all look to be right.
The monitor just occassionally tells me that RPC is unavailable, when I try to ping the server using it's name it isn't there but a ping to its ip address says it is.