SQL Server 2000 running on windows2000 Cluster sometime slow

We are using SQL Server 2000 SP3 on Windows 2000 Advanced Server 2000 SP3. Hardware is a cluster with two servers: Compaq Proliant DL580 with external storage device Compaq MSL 1000(Raid 5).

Since last week we encountered server become very slow on and off (about 5 times) and manually switch over to another cluster node. After switch over to another cluster, become normal.
Normally, our CPU usage is 10 to 30% on the server. When server slow, CPU usage become 50% and never go down.
We have checked with HP and they said nothing wrong with Hardware. Please advise us how to troubleshoot source of the problem