Sometimes I cannot access through RDP

Sometimes I cannot access a Windows Server 2003 computer through RDP
(through LAN).

"This computer can't connect to the remote computer
Try connecting again. If this problem continues, contact the owner of the
remote computer or your network administrator."

I can ping the server, and access shares still.

The only thing I can do is reboot the server using a command line program
from Systernals, called PSSHUTDOWN. After rebooting, I can access the
server as normal through RDP.

What is causing this occasional problem??

Thanks

ss.