Nrouter cause 100% CPU in Windows 2000 Server

I have a server with Windows 2000 Server SP4 (all updates applied) and Domino 6.5.1 and when the server recieved the messages sequence the processor show 100 % of CPU utilization (nrouter.exe on the top). My server have 1GB of RAM and 1 PIII 1 Ghz (256Kbof cache). My ‘Pending mail’ gets 500 messages and my server is slower!!!What is this?

Thanks