Odd server activity

[ThatRickGuy]

Hey Guys,
I'm not a DBA, so this kind of stuff is way out of my realm of knowledge. We have a server that is running 3 seperate Oracle instances. Most of the time, everything is just fine, but every once and a while, the three instance will start taking turns pegging the processor. Average load regularly is insignificant, but once they start doing what ever it is they are doing, one of the three will us all available processing power for about 15seconds, then it will drop down and the next instance will take all available processing power, etc... The three instance will just keep doing this until we kill and restart the services.

Any idea what would cause this issue? Or where to look for clues to the cause?

-Rick

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[dbtoo2001]

http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/technique.htm#sthref229

http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/autostat.htm#PFGRF02606