Windows Server 2003 freezing problem

Hi everyone,
I have Windows Server 2003 and Microsoft SQL Server 2000 runing on IBM X3650 server(6GB of ram,quad core, 5×120 GB SAS disks in raid).Server was working fine until one month ago when it started to freeze. It simple go offline ,freez screan,become complitley unresponsive. When this hapens I must restart machin manualy.
Server was controled by IBM servis and they havent found anything.
Doese enyone know what could be the proble.Any sugestion is welcome.
THANKS